It all started one morning when my engine almost cut out as I rounded a curve going up a steep hill. Slowing to a virtual crawl -- several turtles waved as they whooshed by -- I somehow made it home, chugging on empty.

It was then that I remembered reading somewhere that we should think of our bodies as cars. An older model, say a station wagon, may need a boost to make it up those hills.

My body is definitely the station wagon type (I picture it as baby blue with a few rust spots and dented fenders), and on that day, it felt like a stationary wagon with more than rust spots and dented fenders as the problem.

Sure, I'd been pushing it a little further and a little more uphill and in hotter and more humid conditions than usual, but for that, after all these years and all those easy miles, my trusty wagon was going to stop on me?

I was appalled. Come on body, I said, in one of those heart-to-hearts you have as you coax a horizontal self -- feet lolly-gagging way behind the head -- up the hill. Come on, I said, we can do better than this.

My companion, I noticed, was zipping up those hills. So what if her model is more compact and about 10 years younger than mine? Something had to be done.

The problem was not lack of water. I've read all those articles about the importance of drinking water, especially in the heat and humidity. I took hydration so seriously I carried water with me on five-mile runs. First I clutched my precious fluid in a small bike bottle, which I could also squirt over my head during particularly steamy stretches. I alternated carrying it in both hands and figured I was also giving my biceps a mini workout, albeit detracting considerably from the freedom of a good, unencumbered run. And I must have been quite a sight.

But then I invested in one of those holsters, a bright blue one, in which a well-cushioned water bottle just rides comfortably along on your derriere, or wherever you choose to put it, and my arms, blessedly, were free again.

Still, water was not enough for this engine and those hills and this heat. I began experimenting with different fluid replacement drinks, sipping some about every 15 minutes or so on the run. The frequent quaffing didn't help my already slow 10-minute-mile pace -- drinking takes time -- but it sure helped my endurance. While the old station wagon wasn't exactly roaring up those hills, it was purring along nicely, thank you. I didn't care if it were a placebo or not, it was working.

Which brings me to the reason for this story: to encourage you, if you're a vigorous exerciser, to experiment with different fluid replacement drinks. Again, your own body is your best laboratory, and what works for me may not work for you, and vice versa.

The other thing you must know: Water is no longer considered the superior beverage before, during and after exercise.

Although it was once believed that water was absorbed into the bloodstream more quickly than electrolyte and energy replacement drinks, recent research shows this is not true.

"The body will rehydrate more quickly with sports drinks," says physiologist Ethan Nadel of the John B. Pierce Laboratory, Yale University School of Medicine.

The drinks not only immediately replace sodium lost through sweat, he points out, but also help to retain water in the body. (Remember when you're on a diet you're told to limit your salt because it retains water?)

University of Iowa exercise physiologist Carl Gisolfi's much noted research with cyclists also shows that electrolytes (chiefly sodium and chloride) speed the entry of water into the bloodstream, hastening recovery.

"In terms of prolonged exercise {90 minutes or more}," he says, "a carbohydrate and electrolyte beverage is superior to water.

"We can no longer support the idea that water is the best fluid replacement. Water is a good fluid, but it's not enough in prolonged exercise."

Whereas Gisolfi is careful to stress "prolonged exercise" as the barometer in determining whether or not sports drinks are necessary, other researchers have a why-not attitude. Why not, that philosophy goes, replenish your body with carbohydrates and electrolytes, even after 15 minutes of vigorous exercise?

And when pressed -- if you were carrying only one bottle on even a brief outing rigorous enough to work up a sweat, should that bottle be filled with a sports drink? -- the experts say yes.

"All things considered," says Gisolfi, "you might be better off with that."

Consider also this claim from a recent issue of Runner's World: "Athletes given sports drinks during exercise ran as much as 20 to 30 percent farther than they could without drinking them and significantly farther than they could drinking water."

But which drink? Which one should you choose from that proliferating and confusing array staring at you from the shelf in your health-food store? Or, increasingly, grocery and convenience stores?

Basically, say the experts, You be the judge. You may or may not be able to tolerate fructose, glucose or sucrose and may need, for example, the more complex glucose polymers to avoid a sugar high and low.

Only your body knows what works for you. Try out different drinks in training, but never for the first time in competition.

"There are marked individual differences," says Gisolfi, in the toleration of different beverages. Most people, he says, can tolerate a basic 6 to-8 per cent carbohydrate solution. Different sugars, however, affect different people in different ways.

One place to start experimenting may be with the giants: Gatorade (from the Quaker Oats Co.) and Exceed (by Ross Labs, makers of the infant formula Similac).

Gatorade has long been the biggest seller and the choice of 90 percent of the market -- "I love its taste," says my mailman -- whereas Exceed, right or wrong, has occupied a more lofty position as the drink of serious athletes.

But you won't get scientists -- or at least I couldn't -- to publicly endorse one over the other. Again, they stress the importance of individual testing.

"They both have scientific grounding," says Gisoldi. He adds, however, "I don't think we've formulated the ideal solution yet."

A recent setback in the Exceed camp: Gatorade got the contract for the Iron Man competition. There are plans, however, to make Exceed available on a more mass-market level similar to Gatorade.

How much should you drink? During very strenuous activity, say the experts, about 6 to 8 ounces every 15 minutes. Remember, you're keeping up the water level in a body that's about 40 quarts water.

And all this doesn't mean you should stop drinking water. No one disputes the wonders of water; it just needs a little spiking once in awhile. Some triathletes alternate drinking from two bike bottles, one with a fluid replacement and energy drink and the other, plain water.

Also important: Your drink should taste good, which means you'll drink more. And that makes everyone happy.

You can't, it seems, drink too much. "The wisdom of the body prevails," says Nadell.

The wisdom of Frank Shorter's body informs him that a defizzed cola provides a better boost--about a 10 percent carbo jolt--than the energy-replacement drinks on the market. He drank the flat cola when he won the gold medal in the 1972 Olympic Marathon and still does today. He also goes with just plain water on some long runs.

"Whatever you use, you have to practice it in workouts -- get your body used to it -- before you use it in competition. Get a consistent routine, and pay attention to your own psychological placebo situation.

"And remember, we're all very susceptible to any idea of a quick fix -- be it new socks or a different drink -- before a big event. No. Uh, uh. Don't try it then."