If you are a Runner, a Marathon runner, a Rambler or a Hiker.

Picture a blazing 90ºF Sunday in July. Your basic instinct tells you to lie panting in the breezeway with the dog, or poolside with a cold drink. But your so-called "higher reasoning" tells you that your success in October's marathon hinges on whether you complete today's long run. What's it going to be?

Never mind, you're a runner, so we know how you're going to answer that. Which means you need to know this: Drinking an adequate amount of fluids is the single most important thing you can do before, during, and after a run. Especially a long run on a hot day.

So grab a cool drink, find a comfortable place to sit, and listen up.

1. Drink early and often--every day. With dehydration, a drop of prevention is worth an ocean of cure. Drink water or other low-calorie fluids in small, steady quantities throughout each day, to the point when your urine flows clear. Bring a water bottle with you wherever you go, or take a drink every time you pass a water fountain at work or school. Staying well-hydrated throughout the day benefits you in ways beyond your running. It helps keep you alert and will prevent that dull, headachy feeling that slows you down in the middle of the day.

2. Fortify yourself with fluids before you run, and aggressively consume fluids during your run. An hour or two before your run or race (depending on your tolerance), top off your fluid tank by guzzling 16 ounces of water or sports drink. Then take in between 5 and 12 ounces of fluid every 15 to 20 minutes during the run. If you're running for less than an hour, you'll be fine drinking just water. Your body's stores of carbohydrates and electrolytes are sufficient to carry you for at least that long.

3. When you run long, use a sports drink. Study after study shows that for exercise lasting more than an hour, runners perform significantly better when drinking a sports drink than when drinking plain water. Sports drinks contain carbohydrates at a concentration of about 7 percent, which, by the way, is significantly lower than that in your average soft drink. This is good, because the lower concentration enables you to absorb the fluid more quickly. Its carbs provide fuel, while its electrolytes--sodium and potassium--stave off cramps, nausea, and hyponatremia (a dangerous condition caused by drinking too much water). Go with a flavor you like as this will encourage you to drink more.

4. Don't wait until you're thirsty to start drinking. By then, it could be too late. In other words, the old tenet of "listening to your body" doesn't work when it comes to fluid replacement. We often don't feel thirsty before or in the early stages of a long run, but that's exactly when we need to start drinking. Otherwise, dehydration can set off a chain of negative reactions. Since sweat is made from fluid in your blood, your blood becomes thicker as you sweat and your heart has to work harder to pump. Your body temperature rises, which creates the need for more water. Meanwhile, dehydration compromises your body's ability to absorb carbohydrates and electrolytes from your digestive tract. And so on.

5. But don't drink too much. If you feel or hear sloshing in your stomach, its telling you it's full, and you don't need to drink for a while. You might have to monitor this carefully, though, because we all have varying rates at which fluids leave our stomachs and enter the rest of our systems. Variations in size, gender, age, and metabolism come into play. You might not slosh; you might cramp or feel nauseated instead. Whatever signal you receive, stop drinking for at least 15 minutes.

6. After your run, drink between 16 and 24 ounces of sports drink for every pound of body weight you lost during exercise. This means you'll take in more than you lost, but that's okay because you'll urinate some of it away. Again, make sure you consume sports drinks or other fluids that contain sodium, which will help you retain the fluid you drink.

7. Beware of hyponatremia, a potentially fatal condition caused by taking in too much water and too little salt. Hyponatremia, or "water intoxication," usually happens only during long, hot runs, when a runner loses a lot of sodium through sweat and consumes a great deal of plain water. This combination may unwittingly dilute sodium levels in your blood, which sets off an electrolyte imbalance. Hyponatremia can trigger seizures, coma, and even death. Initial warning signs mimic those of dehydration, including confusion, disorientation, muscle weakness, and vomiting. Women and slower, beginner endurance runners are at most risk of this condition. Still, the likelihood of developing hyponatremia remains small. Banish hyponatremia from your worry list by 1) including salt in your normal diet (particularly a few days before a long event), and 2) drinking sports drinks, rather than plain water, on runs over an hour.

8. Be careful out there. The better your overall condition, the better you'll be able to cope with low or moderate degrees of dehydration. (Case in point: In the 1984 Olympic Marathon, Alberto Salazar lost 8.1 percent of his body weight in sweat, and still ran a 2:14.) But if you feel yourself slipping into fluid debt-symptoms include chills, dizziness, disorientation, and cessation of sweating-don't panic. Stop running, find shade or an air-conditioned building, and start drinking as soon as possible. If you don't quickly improve, ask for help or call 911. If you do start to feel better, resist running and walk or ask for a ride home. Then vow to never let yourself become dehydrated again.

Don't Let This Happen to You

Performance starts to decline when you lose 3 percent of your body weight in sweat. For a 150-pound person, thats 4.5 pounds. Beyond 3 percent, performance falls off even more sharply. Dehydration becomes a life-threatening condition when it reaches 15 to 20 percent of body weight, usually through illness.

Right This Weigh

Try this method for measuring how much water you should drink during and after your runs:

Immediately before and after several upcoming runs (especially those in hot weather), weigh yourself without any clothes on. If you lose 1 pound, this means you sweated approximately 16 ounces of fluid. Two pounds means you sweated 32 ounces, and so on. If you lose a pound consistently during your 30-minute runs, you need to replenish your fluids at a rate of 16 ounces per 30 minutes of running.

If you find you can't drink enough to offset your sweat loss no matter how hard you try, don't worry. Just make sure to "overdrink" once you finish your run. That is, if you lose a pound, you should drink one-and-a-half times your normal replenishment amount, or 24 ounces. This beverage should contain sodium, which will help you retain the fluid better.

Be in the Minority

Dehydrated athletes fatigue significantly faster than those who stay hydrated, yet the typical runner replaces less than 50 percent of his or her sweat during exercise.

According to Scale

Officials at the Western States 100-Mile Endurance Run in California actually lug bathroom scales into the Sierra Nevada mountains, and place them at the aid stations. Why? So runners can weigh themselves during their trek, enabling them to balance their fluid intake with their fluid loss. This low-tech method remains the best way to figure out how much you need to drink while running.

DRINK THIS