When I first read The Yage Letters, William S. Burroughs' cackling account of his drug search in Peru and down Colombia's Río Putumayo to locate what he referred to in Junky as the grail of psychotropics ("Yage may be the final fix") – a trip in which he was rolled, robbed, starved, diverted, and endlessly bullshitted in his quest to find a high that towered way beyond your average stoner's dreams of doobage – I closed the book and thought: I really must repeat his trip sometime.

Yage is yajé, Banisteriopsis caapi: vine of the soul, secret nectar of the Amaéon, the shaman's holy drink, the ultimate poison, a miracle cure. More generally known as ayahuasca, a word I found bewitching, it was said to make its users prescient, if not telepathic. Rocket fuel is another active ingredient: In an ayahuasca trance, many users have testified, you travel to distant planets, you meet extraterrestrials and moon goddesses. "Yage is space time travel," Burroughs said. A singular proof of this is the collection of trance-state paintings by one of ayahuasca's greatest proponents, the shaman and vegetalista Don Pablo Amaringo. Ayahuasca Visions, Don Pablo's book (written with Professor Luis Eduardo Luna), is a meticulous pictorial record of his many ayahuasca sessions. But there are risks in the drug too, not the least of which are convulsive fits and ghastly spells of vomiting. Many of Don Pablo's paintings include an image of someone engaged in picturesque puking.

Years passed. Then I was in the middle of a novel, and stuck for an idea, and in this period of Work in Stoppage I remembered "The Aleph," the great story of visions, by Jorge Luis Borges, where a man finds the inch-wide stone, the Aleph, that allows him to see to the heart of himself and the world. I realized the moment had arrived for me to find the insight and telepathy of ayahuasca, which would be my Aleph.

Some friends, former amigos of the old gringo and self-exiled writer Moritz Thomsen, told me they knew of ayahuasqueros among the river people in eastern Ecuador. I was given the name of an outfit that shepherded aliens into the tributaries of the upper Amazon, where traditional healers abounded. I made the arrangements, and soon found myself in a cheap hotel in Quito, awaiting the arrival of the other travelers on this drug tour.

"Drug tour" was my name for it. "Ethnobotanical experience" was the prettified official name for it, and some others spoke of it as a quest, a chance to visit a colorful Indian village, a clearing in the selva tropical, where just a few decades ago American missionaries sought early martyrdoms among the blowguns and the poison-tipped arrows of indignant animists resisting forcible conversion to Christianity.

The people who organized this drug junket characterized it as a high-minded field trip, eight days in the rain forest, for eco-awareness and spiritual solidarity, to learn the names and uses of beneficial plants. One of those plants was ayahuasca. There was no promise of a ritual, yet heavy hints were dropped about a "healing." We would be living in a traditional village of indigenous Secoya people, deep in Ecuador's Oriente province, near the Colombian border, on a narrow branch of Burroughs's Putumayo, where the ayahuasca vine clinging to the trunks of rain forest trees grows as thick as a baby's arm.But I had a bad feeling from the beginning. I am not used to traveling in groups, and this was a nervous and ill-assorted bunch, eight or 10 people, a larger number than I had expected. The great attraction for me – it was the reason I had signed up – was that Don Pablo Amaringo would be our vegetalista. But even Don Pablo, in his stirring lecture in Quito before we set out, spoke of the conflicting vibrations he felt among the people in our group.

Don Pablo's gentle manner, his shy Amazonian smile, his wide knowledge of jungle plants, made him instantly persuasive. He was golden-skinned and slight of build, and his expressions were so animated and responsive it was impossible to tell his age. He had as a master painter been able to capture the ayahuasca experience in his pictures. He is a respected shaman, though he seldom used the word. Shaman is a term from the Siberian Evenki people that has gained wide acceptance. In Quechua the word for shaman is pajé, "the man who embodies all experience."

Don Pablo was also a teacher; he ran an art school in Pucallpa, Peru. In 1953 Burroughs had found ayahuasca in Pucallpa. I trusted Don Pablo from the moment I met him. He remains one of the most gifted, insightful, and charismatic people I have met in my life. Don Pablo correctly diagnosed that I had unfinished business back home – my wife unwell, my affairs in a muddle; he seemed to know I was stuck in my book. His shrewdness reminded me that a substance named telepathine had been isolated from ayahuasca.

"Your mind is partly here and partly at home," he told me.

The others disturbed me. Except for a psychiatrist-poet and a young man who was on the trip to add a chapter to his book about his drug experiences (not long before, he had been roistering at the Burning Man festival), these people were not travelers. Even in Quito they looked out of their depth, and later, as we penetrated the Ecuadoran interior, they seemed to wilt. One woman cried easily, one man proclaimed militant Judaism, another woman her spirit search; a man confided to me that he was on a quest for spiritual fulfillment, another man sobbed, "I need a healing." One lovely girl was beset by a chronic case of the squitters.

They thought of themselves as searchers. They seemed to have a touching faith in the efficacy of this trip, yet they seemed abysmally ill-prepared for its rigors. The sobbing woman did not bother me much; I was more concerned by the anxious screeching facetiousness of some of the others. They seemed to me innocents, they were easily spooked, yet looking to repair their lives. Most had never been in a jungle before, or slept rough. They looked muddled, giggling desperately in sweaty clothes, as though expecting to be ambushed. The organizers did their best to soothe the nerves of these people; yet I remained querulous and discontented, unused to so much apprehension. One woman was menstruating: The ceremony was forbidden to her.

Finally assembled, we left Quito late; we procrastinated at the Papallacta hot springs. Idling there at the edge of the forest, Don Pablo showed me a blossom called angel's trumpet, of the brugmansia family. There are many varieties, but this one was especially potent. "They call it datura – toé in Guarani. It can give you visions. In some ways this is more powerful than ayahuasca."

"In what way?"

"Great visions," he said, rubbing a leaf the way a Chinese connoisseur evaluates a piece of silk, "but it can make you blind."

Night fell as we traveled east, going slowly on bad roads. We arrived in darkness at Lago Agrio, a boomtown that had grown to accommodate the sprawl of the American oil companies, which were exploiting the rain forest and displacing the Indians. At the hotel we took pains to hide our bus ("Or it will be stolen"). We went to sleep in the stinking town of furtive shadows and sharp clicking heels; we awoke in a hot, bright place, a confusion of traffic and the sour creamy stink of spilled oil and the toxic-saturated earth.

Lago Agrio was a blight in the harsh equatorial sun. Because of a delay in our departure for the river, I lingered over coffee and fell into conversation with Joaquin, a local resident and volunteer guide who claimed to be a vegetalista. He was a young man, not more than 30, with the look of an ascetic – long hair, faded shirt, sandals – that was also the look of a risk-taker. He told me that the noises I had heard all night were the scurryings of prostitutes. It was, he said, a town of whores, drugs, gunrunning, rebels, and oil prospectors. You could buy anything here, at any time of the day. Even the whorehouses never closed. It was then 8:30 in the morning.

"The burdeles are open even now!" Joaquin said.

I challenged this, so he took me on a 10-minute taxi ride to a low building on a dirt road. Inside, women old and young, all of them in bathing suits, sat primly on folding chairs in front of little cubicles that surrounded a large dance floor. No one was dancing, though the music was loud. Two men were fighting, knocking over chairs. Eight or 10 other men were drinking beer. The morning sun slanted through the building's small windows.

"They work all night in the oil fields and come here in the morning to get drunk and find a woman."

Joaquin led me through the backstreets of the ramshackle town, where in little shops merchants whispered and handed me bones. "Endangered species!" The polished skulls of jaguars – called tigres – were for sale. There were also hunks of tortoiseshell, stuffed bats, mounted lizards, dead spiders transfixed by needles, and weapons of all sorts – blowguns, poison darts, machetes, wicked-looking shivs, bows and arrows.

"This was once rain forest. Just Indians and animals." Joaquin asked me what I wanted. I could have anything – a monkey skull, a tiger skin, drugs, guns, a 14-year-old girl. He could even arrange what he called a Toxic Tour, a survey of the local blight he said was caused by Halliburton and Occidental Petroleum.

I told him I was going down the Río Aguarico with my group of gringos, to a village of the Secoyas. He recogniéed this as shorthand for a drug tour, and he made an elbow-bending motion and a drinker's gesture.

 "Ayahuasca," I said.

"You could drink it near here. I know people," he said. And he showed me in another shop bags of medicinal herbs and plants, and fat, dusty lengths of cut-off ayahuasca bulging in gunnysacks.

"No, I want to see the village."What had started as a fairly straightforward search for the ayahuasca experience was becoming more complex, crowding my head with images: the oil squirting from bandaged pipes running alongside the road, the faces of the prostitutes – young fearful girls, old resentful women, the devilish faces of their customers – and the grinning tiger skulls, the spiders as big as my fist, the heat, the dust.

And terrorism: Joaquin had told me that the previous night on the bridge into Colombia, just about 10 miles away, some guerrilla soldiers of the FARC had stopped 20 cars. At gunpoint they had given the drivers cans of gasoline and said, "Douse your car and burn it, or we'll shoot you."

Twenty flaming cars blocked the San Miguel Bridge to Colombia, at La Punta, the frontier, that day.

"It is to discourage visitors," Joaquin said, with Ecuadoran understatement.

Leaving Joaquin, I rejoined the ecotourists. We took a bus to the muddy settlement of Chiritéa on the banks of the Aguarico. In Lago Agrio, on the roadsides, in Chiritéa, and along the riverbanks were mud-spattered signs, all bearing the same message: prohibido el paso. Keep out. We then boarded a dugout canoe and crouched inside this enormous hollowed-out tree trunk and set off downstream, powered by a farting outboard motor.

The river narrowed from a hundred yards or more to 50, then to 30, in less than an hour, the jungle overhanging it like thatch – drooping bamboo and trailing vines and big-leaved trees. The nervous chatter of the passengers in the dugout drowned the cries of flitting birds.

Such a river, deep brown from the silt of the runoff of the rains, and such a fragile-seeming boat, in such a distant place, created a sense of uncertainty among the gringos. The anxiety of traveling slowly down the gullet of the jungle suggested that a place so hard to get to would be equally hard to get out of. We were in the hands of the monosyllabic guides and the taciturn boatmen. I did not like the feeling of being in the same boat with these others. I need a degree of control over my coming and going. I am not happy in a herd, especially a herd of debutantes.

Daylight drained from the sky, the jungle darkened, the river gurgled at the hull of the dugout; yet the river, amazingly, was still visible, holding the last of the light, as though the day glowed undissolved in its muddy current.

"Remolino," a boatman said. "Whirlpool."

Beyond that swirl, and a long reach of the river, was the village: men in orange smocks, one or two wearing coronets of feathers and vines, boys snatching at the bowline and helping the visitors ashore.

We were directed to a communal platform, where we would all sleep on mats or in hammocks. I resisted this, partly because furry knuckle-size insects were bumping and batting the glaring lanterns, but mainly because I wished to sleep alone. I had brought my small tent, a Moss Starlet – packed, it was the size of a football – and my Marmot down-filled sleeping bag, much smaller in its bag than the tent. I set up camp in a clearing at the edge of the village.

For the following two days the creepy feeling I'd had at the outset deepened. I felt an uncertainty awaiting me back home, a sense of misfortune and dread; and also a disarray, a greater uncertainty here. The awareness of killing time wore on me in the sadness and decrepitude of the Secoya village.

I sat on a fallen log with Don Pablo, making notes, while spiders and ants crept across the pages of my notebook, and the river sucked at the muddy bank. I told him about the trouble with my novel. He spoke to me about the Eye of Understanding.

"This eye can see things that can't be seen physically," he said. "Some people have this Third Eye already developed. And for others the Eye of Understanding can be acquired through ayahuasca or some other certain jungle plants."

Each morning the same question. "Tonight?"

"Not tonight."

Not auspicious, or was it that a certain shaman had not arrived as planned, or that signals had been crossed? A great sleepy uncertainty dank with the moss and mildew of the forest settled over us.

If someone seemed at a loss for something to do, he or she was told, "You can weed Juana's garden."

Or we could paint pictures, or help build one of the structures, or consult the healers on botanical strategies. Most were happy to pitch in, but impatience was growing; a sense of discomfort, disorganization. The gringos who had seemed so tidy in Quito were looking grubby, sweaty, careworn. The Frenchman ridiculed America, the young writer objected to his casual abuse; a woman described her life as a series of sorry episodes and began to cry. A low level of bickering began as a barely audible hum in the jungle clearing.

"Where have you been?" people began asking me.

"Looking around," I said, annoyed that my absence had been noticed. In fact, I was spending time on the riverbank at the edge of the village making notes, or in my tent, away from the spiders, listening to my shortwave radio.

One morning Enrique, an Ecuadoran man, was denounced for his drunkenness the night before. As he was being humiliated and asked to apologize before the gringos, I smiled at his accusers' sanctimony.

When they were done, I pointed out that all this man's persecutors were chain-smokers and drug users. What was the problem with alcohol?

"Alcohol has taken a terrible toll on the indigenous people here," one of the American guides said.

And I was also thinking: Where's the ayahuasca? Don Pablo went on explaining it to me. Ayahuasca was like death, he said. "When you drink it, you die. The soul leaves the body. But this soul is an eye to show you the future. You will see your grandchildren. When the trance is over, the soul is returned."

One day, bored and restless in the village, I found a Secoya man to take me deeper into the rain forest.

He said, "We can see flowers. Birds. Big trees."

Preceding me, he slashed with a machete; a small Secoya boy followed. This was like Burroughs's trip, just as aimless and improvisational. People went on such drug tours in a mental quandary, it seemed. They were unused to being at close quarters in a simple village, and they were growing impatient waiting, as I was, for the shaman to summon us to the ayahuasca ceremony. I was happy to be away from their agitated laughter.

We walked for three hours in the humid heat on a muddy track under the high rain forest canopy. The flowers I saw growing wild were flowers I associated with Hawaii: brilliant heliconias, beaky strelitzias, wild-eyed blossoms, pink torches of wild ginger, and the attenuated Datura brugmansia, angel's trumpet, that gave people visions and made them blind. Ayahuasca, too: The vine was unprepossessing and serpentine on the tree trunks.

Only the dimmest daylight penetrated to the bottom of the forest. The greenish air was littered with gnats and filtered sunlight, and here and there a large woolly wheel of a spider's web, the spider crouched at the edge like a small dusty plum with legs.

Just as I was thinking it was possible to believe that, though humans had passed nearby, none had interfered with it, nor had ever bent a stem, nor plucked a flower, that this was a little Eden of the Secoya people, the small boy called out, "Escucha," tilting his head to listen.

There came a far-off chugging, like a motorboat plowing invisibly through the sky, and when it drew closer it became a more distinct yak-yak-yak.

"Mira! Helicoptero," the boy said, his hair in his eyes.

A shadow like a big brown cloud passed overhead, a gigantic Russian helicopter.

The forest dome, with its branches and leaves, prevented us from seeing the progress of the helicopter, yet we still heard it and were able to follow the airship's percussive sound, the drumbeat of its engine burps in the distance.

We were off the path now and chest high in ferns and big leaves as we saw ahead a brightness, perhaps a clearing, and then the descending shadow of the helicopter settling to earth.

We were stopped by a head-high chain-link fence that ran through the forest, razor wire coiled along the top edge, and skull-and-bones signs lettered in red, prohibido el paso, every 20 feet or so. Sunlight scorched the clearing within the fence – sunlight and steel towers and boxlike prefab structures and oil drums, and the huge sputtering helicopter, its twin rotors slowing, as men in yellow hard hats rushed back and forth from its open cargo bay, unloading cardboard boxes.

The encampment was entirely encircled by the fence and the forest. No road led here. And there was no break in the fence – no opening, not even a gate. When the sound of the helicopter died down we could hear the softer but regular pulsing of an engine and could see a steel cylinder moving up and down in the center of the clearing, pounding the earth, with gasping and swallowing noises, and the lurch of unmistakable grunts that sounded like squirts of satisfaction, pumping oil.

Near the entrance to one of the new bright boxy buildings, an Ecuadoran all in white – white shirt, white apron, tall white chef's hat – was conferring with another swarthy man in a short black jacket and striped trousers and a bow tie. This second man, obviously a waiter or a wine steward, held a tray on his fingertips, and on the tray a pair of thin-stemmed wine glasses and a wine bottle in an ice bucket.

Gringos were climbing out of the cockpit of the helicopter – clearly Americans.

"Petroleros," the Secoya man said, and added that we must leave at once.

It was one of the ugliest things I had ever seen in my life.

"This is Secoya land," I said. "How can they be drilling for oil?"

"We own what is on top," he said. "The government owns what is under the ground."

Later, I learned that the local people had been paid a pittance by an American oil company so that the fence could be erected, but no profits would accrue to them, and it was only a matter of time before this part of the rain forest would have the shops and brothels and bars and oil-spattered roads of Lago Agrio.

The vision of this oil well in the virgin forest added to my sense of derangement and demoralized me. I consulted Don Pablo.

"You are not calm," he said, and held my hands.

This was an understatement. I crawled into my tent that night, listening to the chatter of the gringos on the sleeping platform, wondering whether I had the stomach for this. My search for the final fix was turning into a muddle of procrastination. That night I had a nightmare: my wife very ill and calling out for me. In the morning I put this down to ambiguous guilt, my unconscious mind justifying my confusion and apportioning blame.

Sitting on the riverbank pondering what to do, I saw three gringo women from our group dressed in shirts and shorts begin swimming across the river from the far bank. They were chirpy, gargling water as they clumsily paddled in the swift brown stream. One cried out, "I lost my ring! It just dropped off my finger!"

The two others hesitated, and as they stopped swimming they were pulled downstream. The woman who lost the ring said, "Never mind. It was meant to be," but the river was too much for her, too. I kicked off my sandals and dived in, reached her after a few strokes, and brought her to shore. One of the others was thrashing, and didn't need much help, so I went after the third, who was heading toward Brazil in the churning current.

She was blowing and gasping as I got to her. Her clothes were dragging her down, she could barely lift her arms, but her shirt gave me something to hang onto and so, slowly – cautioning her not to grab me: I feared her panicky grip – I tugged her to shore.

Perhaps she was in shock. She whinnied a bit, mirthless laughter. She didn't thank me. She said, "I think I could have made it on my own."

In that moment of ingratitude, near-tragedy, and plain foolishness, I decided to bail out. My bad feeling about this group and this place seemed justified. What was I doing here? I had come for the drug, and I had seen the horror of Lago Agrio – whores and drugs and stories of burned cars and the Toxic Tour. Looking for the purity of the jungle I had found the violation by the oil people. These reckless women who had almost drowned themselves seemed proof that worse might be in store. And besides my wife was ill.

I rolled up my sleeping bag, folded my tent, and found a Secoya man who said he had a boat with an outboard motor. I paid him the $100 he asked for – pretty steep, I thought – and he took me upriver to Shushufindi, where I found my way back to Lago Agrio.As soon as I was alone I could think straight. Instead of eco-chic, ethnobotany, the rain forest experience, shamanism, or visions, I had encountered child prostitutes, gunrunners, oil people, blighted jungle, the place surrounded by guerrillas of the FARC; and the diminishing number of Secoyas seemed doomed. That village would soon be swallowed by the encroachment of oil people who were only half a day's march through the forest.

Perhaps this was meant to be my adventure, though I had not known it at the outset. The whole point of adventure is that it is unplanned; a leap in the dark, verging on the unfortunate, offering glimpses of danger; and what separates adventure from disaster is that you live to tell the tale.

In Lago Agrio I found Joaquin. He made the swigging motion with his hand and arm and gave me an inquiring smile.

In Spanish, I said, "No. It's a Chinese story."

Meaning, as the idiom has it, long and preposterous.

"Maybe I can help you," he said.

The others downriver were perhaps preparing for their ceremony. I had come all this way, and yet sitting there in that dreadful town I felt calm, even happy. I was on my own, I called my wife – yes, she had been ill, but with worry because she hadn't heard from me; so I had a reprieve – and now I had no sense of urgency.

My ayahuasca ceremony was private, one-on-one, in a large, open-sided shed inside the walled compound – prohibido el paso – of a large house at the edge of Lago Agrio. Don Pablo had briefed me and so I was prepared – though I would have preferred him to be my shaman. After all I had seen, I feared this ceremony would be an anticlimax. This was not a Secoya village, yet the shaman was real enough, and as for the ayahuasca, my barfing convinced me of its potency, the way poison can be medicinal.

Crouched and retching, I slipped into a time warp, twisting in a hammock to an auditory gust, a cataract of sound, of tinkling song, and matching images – torrents, and a waterfall of snakes, the serpents slithering in lakes of oil, the bleeding trees and spiders, helicopters that could have been spaceships, the scaly greasy river, which bunched and swelled, like an anaconda. The rasping hum in my belly might have been the chanting of the shaman. My whole body was vibrant with a syncopation of grunts and murmurs, but the colors I saw were subdued, like enlarged pixels.

The visions, though disturbing – the sticky oil flow, the twisted snakes – did not strike fear into me, but rather seemed to be part of something whole and coherent, fitting into a harmonious world of creation and destruction. The harmony was both the sound of the chanting and the glitter of the foliage and flitting birds and, as in a poem by Rimbaud, monstrous mouthlike blossoms.

I woke drooling and gasping, somehow on the floor of an open-air pavilion, my face stuck to a raffia mat.

Back home, I was reassured that my wife was fine now. For the whole of my time downriver she had felt short of breath, almost asthmatic with the anxiety that I was in danger. Some time later I talked to some people from the tour, who had joined the ayahuasca ceremony in the Secoya village. On one or two of them the drug had had no effect; others had experienced the moon shot. And there was a visitor – me, in my panama hat and Hawaiian shirt.

"Two people saw you," one of the ayahuasqueros told me. "You were there, in our visions, watching us."

That ghostly visitation was like a metaphor for being a novelist. Not long afterward I threw away what I had begun of my novel and started the real thing. Having experienced the blinding light of this drug tour, I felt I understood my subject. My novel went well, and in the several years of my writing it I often thought: Sometimes there's a spider in your cup that you don't see. You drink and move on, never knowing the creature is lurking in the liquid. But I had guzzled the whole cup and had a glimpse of the poisonous insect. I was resolute, with one of my favorite lines from Shakespeare murmuring in my mind: "I have drunk and seen the spider."

Truly, I had not known what I was in for before I went. I now knew where, and what it had done to me. "A place where the unknown past and the emergent future meet in a vibrating soundless hum," Burroughs wrote of his ayahuasca experience in The Yage Letters. "Larval entities waiting for a live one."

That was a lyrical way of putting it. And I had been the "live one." Ayahuasca was the formal reason for my journey, but the whole point of a leap in the dark is that you cannot foretell your fate. Many things I had seen on this trip had mattered much more than the ayahuasca. A few gringos dabbling in psychotropic drug potions was just a charade compared with everything else that was going on beyond the village.

In a sense the effect was like that of datura, the plant that makes you blind. The systematic oil search and the frantic drilling amounted to a conspiracy by American oil interests as they connived with the Ecuadoran government to take the oil and change the face of the rain forest forever. The result was – what? Enough oil to run Los Angeles for a week or so, vast profits for a few people, and more hookers, gunrunners, guerrillas, and homeless people in a bigger sprawling Lago Agrio. It was a terrible vision to take home, but one I went on living with. As in "The Aleph," I also saw the circulation of my own dark blood. Adventure is the unexpected experience of discovery, of course; but it is also a kind of death, an end of innocence.