When I staggered across the finish line of the Cowtown Marathon in Fort Worth, Texas, I bravely waved off the race volunteers offering help. Then I puked. And puked again. Within minutes I was lying in the medical tent, hooked to an IV while a first-year osteopathy student learned how to take a rectal temperature. Mine, it turned out, was 103.

As a veteran marathoner--this was my 11th race--I thought I'd mastered the art of avoiding heat exhaustion. Yet three hours in the Texas sun had drained my body of fluids and baked my innards, notwithstanding the Dixie cups of Gatorade and water I'd downed throughout the race. Fortunately, the quick medical attention left me with no permanent scars other than a deep fear of med students wielding thermometers.

Rare is the endurance race where medical tents do not welcome streams of underhydrated souls like myself. And overhydrated. You likely saw the recent articles about the New England Journal of Medicine study that found that 13 percent of finishers in the 2002 Boston Marathon suffered from hyponatremia, a potentially fatal condition caused by drinking too much. With all the conflicting information, you might be tempted to trade in your summer training plans for mint juleps and tubs of barbecue.

Step away from the brisket. We've talked to the leading experts and put together a list of the top 10 strategies for heating up your cardio without (literally) killing yourself this summer.

1 KNOW (PRECISELY) HOW HOT IT IS
It doesn't take a Ph.D. in exercise physiology to know that the hotter the day, the greater the risk of heat illness. But a simple thermometer won't tell the whole story. The Kestrel 4000 Pocket Weather Tracker ($329; www.nkhome.com) will tell you the heat index, a combined measure of heat and humidity. When this number hits 90, keep your workouts short (see tip 7). If it tops 105, head for the pool or stay indoors.

2 FIND COVER
Exposure to direct sunlight can add another 15 degrees to the heat index. I fight the sun by working out in the early mornings, when shadows are long. I also wear a good wicking shirt, such as the Adidas AdiStar short-sleeve top ($45; adidas.com). Not only does it deflect the sun's rays--slowing the rise of my body temperature--but, unlike my nasty old cotton tee, it also wicks away the sweat, which cools me down.

3 BEFORE HEATING UP, COOL DOWN
Heat illness (cramps, heat exhaustion, and, finally, heatstroke) occurs when the body's core overheats. To drive core temps as low as possible, many athletes in last summer's Olympics donned ice vests like those made by Arctic Heat ($175; arcticheatusa.com) before competition. Douglas Casa, director of athletic training education at the University of Connecticut, predicts that precooling is poised to be "one of the hottest" trends in endurance athletics. "It's ethical, legal, and gives you a leg up," he says.

4 DRINK ENOUGH
The best way to determine proper hydration is to measure your "sweat rate." Start by weighing yourself nude. Work out for 30 minutes without drinking under typical hot conditions. Then strip down and weigh yourself again (don't pee until you've weighed in). Figure out how much weight you lost, double it, and that's your sweat rate per hour, in pints. For every ounce you sweat, you should drink an ounce. So if you lost two pounds, now you know that in, say, a 45-minute workout under similar conditions you'd need to drink about 48 ounces. You'll want to drink some every 15 minutes or so rather than all at once. As an added safeguard, some people are also weighing in before every especially hot or long run (including more folks at the Boston Marathon, where contestants have started scrawling their pre-race weights on their bibs). That way, if you feel shaky after you exercise you can step on the scale; if you weigh less, you know exactly how much you need to drink. And if you weigh more, you know you've broken rule numberŠ

5 ...BUT NOT TOO MUCH
You may have noticed that when you go for a run you rarely need to stop to urinate. That's because exercise activates the body's "fight or flight" response, temporarily shutting down the kidneys. That's also why people get themselves into trouble by drinking too much. With the kidneys on hiatus, if they take in more than they sweat out, fluid builds up in their bodies and dilutes their blood; in the New England Journal of Medicine study, even people who drank a sports drink were vulnerable. Still, it's important to understand that in a typical recreational summer run, tennis match, or ride, dehydration poses a far greater risk than overhydration. "To put yourself at risk for hyponatremia," says Casa, "you've got to be exercising and drinking for a very long period of time." In the NEJM study, most of those who over-hydrated took more than four hours to finish the race. If you are out there for an extended period, one good rule of thumb is to always drink on the move; if you don't stop to chug, there's less chance you'll overdo it.

6 CHOOSE THE RIGHT DRINK
Water will meet your fluid-replacement needs during short, relatively easy workouts. But ramp up either the length or the intensity and it's time for a sports drink. With added sodium, these drinks help maintain electrolyte levels, decreasing hyponatremia risks. Plus, they contain sugars that keep your muscles fueled and make you want to drink more, a surefire defense against dehydration. But which sports drink when?

Traditional fluid-replacement drinks
WHEN: During more intense workouts or any session longer than 45 minutes
Look for a drink that contains 6-8 percent carbohydrates (14-20 grams per 8-oz serving). Any higher and the carbs will start blocking fluid absorption; any lower and you risk an energy bonk.

Endurance Formulas
WHEN: During your longest, hardest workouts
These new entries are ideal for serious cyclists, marathoners, and triathletes. As carbs go, they're indistinguishable from the old sports drinks. But they have more sodium, whose importance in stimulating thirst and warding off various ailments grows with the length of workouts.

Recovery Formulas
WHEN: Right after a workout
These post-workout cocktails will help you rehydrate and also offer a quick infusion of protein and carbs. The protein ferries the carbs to muscles, speeding recovery and muscle repair. Look for a protein-to-carb ratio of 1:4 to 1:7, a carb concentration of 6-8 percent, and some sodium to restore electrolyte levels.

7 DO INTERVALS
You heard us right. During the dog days, I often cut my usual workout time--and heat exposure--in half by doing a series of short, high-intensity sprints (broken up by short periods of slow jogging or walking). "And if you get your heart rate up to 160 or 170," points out exercise physiologist Terry Shepherd, "you actually challenge your cardio more than if you do a longer, slower, distance workout."

8 BE CAREFUL INDOORS
On the hottest days you're certainly better off on a treadmill than on baking blacktop, but that doesn't mean you're in the clear. Sure, the gym offers cooler temps, but when elite runner Josh Cox logged a 2:31 marathon on a treadmill last year, he sweated out 7.5 pounds, enough to put him at serious risk for heat illness. Play it safe by drinking often and correctly and by keeping a fan on you.

9 HIT THE POOL
When the heat index tops 90, you could try swimming laps--or strap on the AquaJogger Pro belt ($57; aquajogger.com), and hit the deep end for a session of pool running. It works for 2004 Olympic marathon silver medalist Meb Keflezighi, who uses water running to escape the heat of Southern California and the pounding of the road. To keep your cardio up, try intervals of five three-minute bursts with two-minute "jogs" in between.