Even in wintry weather, heat is as much a threat to joggers as cold.

"Some of the highest body temperatures you'll ever see are in folks running long distances on a cold day," says Larry Rowell, a physiologist at the University of Washington School of Medicine in Seattle. A mountain climber in a down jacket at 20 degrees below zero, he adds, may be in more danger from hyperthermia (overheating) than from hypothermia (heat loss).

"The cold actually helps control body heat," says Dr. Robert Nirschl, director of the Virginia Sports Medicine Institute in Arlington. Marathon runners generally achieve their lowest times when temperatures are in the upper 40s. The real problem, he says, "is getting rid of body heat, because your muscles are working so hard."

But if properly dressed against exposure to wind, cold and precipitation, Nirschl says, most wintertime joggers are under no additional strain from the elements.

Cold itself does not stress the heart or lungs of a healthy runner, experts say. But it may bring on coughing spells in people with asthma or other kinds of chronic bronchial or lung disease. Also, people with hypertension should make sure to take their blood pressure-controlling medicine and warm up before running in severe cold because blood vessels in the skin tend to constrict in reaction to cold air, raising blood pressure slightly.

"It doesn't mean don't run," says Dr. Samuel Fox, director of the preventive cardiology program at Georgetown University Medical Center. "It means warm up more in advance.

"Even walking around the house or standing up and shaving or taking out the garbage is often enough to relax the blood vessels, warm the muscles and increase circulation."

The person who suffers a heart attack while shoveling snow is often cited as a victim of cold weather. But usually that is only indirectly true, doctors say. The heart attack is usually brought on by strenuous exercise, to which the person may not be accustomed, or by an underlying heart condition, rather than the cold.

Nor, despite a widespread belief, is anyone this side of Antarctica in danger of frozen lungs from running outside in winter.

Inhaling extremely cold air can chill the throat and irritate the bronchial tubes, causing temporary asthma-like symptoms called "cold-induced bronchospasms," says Dr. Ed Pavlin, a University of Washington anesthesiologist and a trauma expert, but it won't freeze the lungs. The windpipe divides 23 times in an elaborate branching pattern on its way to the tiny air sacs in the lungs -- enough to warm even subzero air.

"You can get frostbite on the lips," Pavlin says. "But actually freezing the lungs? That's not going to happen."

The trick in cold weather, as in hot, is to balance heat production (from exercise) and heat loss (from skin exposure and perspiration).

"You want to get rid of extra heat but still keep warm," says Pavlin, an experienced marathoner who grew up in Manitoba, Canada, where winter temperatures reached 30 below and even the most devoted runners found that "two miles was about all we could do."

Maintaining that balance depends on many factors -- temperature, wind, precipitation, clothing, warmup, degree of activity, perspiration -- which complicate predictions about the effects of cold weather on exercisers.

"Heat's easy," physiologist Rowell says. "Cold is much more difficult."

For example, the threat of cold usually does not occur during exercise, when the body's work load keeps it warm, but afterward. A runner who overheats and sweats and then stops exercising can chill very quickly. The evaporative cooling effect of the sweat, especially if it is compounded by falling rain or snow, lowers the body temperature rapidly.

Unchecked, that leads to fatigue, slurred speech, uncoordination and other symptoms of hypothermia, a potentially fatal drop in body temperature.

Shivering, the body's effort to compensate for heat loss by making its muscles work harder, is one of the first warning signs of hypothermia. A kind of exercise under duress, shivering can quadruple the body's metabolic rate.

In the short run, that helps rekindle the body's temperature. But in the long run, it is exhausting and exacerbates hypothermia. Shivering should be seen as a warning to go indoors, drink warm liquids or put on more clothing.

In Washington winters, frostbite is rarely a hazard, though cold snaps can make running uncomfortable. Sen. William Proxmire (D-Wis.), one of Capitol Hill's best-known joggers, says he has never come close to frostbite, but adds, "I've gotten mugged a couple of times."

Proxmire says he prefers running in winter cold to running in summer heat.

"There's no danger at all from running in the cold," he says. "When it's cold you do the obvious things. You wear warmer clothing."

Twelve years ago Proxmire jogged the entire perimeter of Wisconsin over several months. By New Year's Day, when he arrived in Superior, at the northwest reach of the state, it was about 20 below and Proxmire was running in a snowsuit and snowmobile gloves.

In Washington, light gloves, a hat and a jacket are the only extra clothing he dons during a cold spell for the five-mile run to his Senate office from his Northwest home. (He rides the subway home.)

Was it ever too cold to run?

"Only in Wisconsin," Proxmire says. "When it's more than 30 below, I say the hell with it." First, Wear a Hat

Experts offer these tips for cold weather exercisers:

* Wear gloves, warm socks and a hat to reduce heat loss and protect the extremities against wind chill. Hats are especially effective, because more than half of the body's heat loss passes through the head, prompting the old adage: "If your feet are cold, put on a hat."

* Keep dry to stay warm. In rain or snow, wear a water-repellent or waterproof jacket. Avoid puddles and fallen snow, which soak the shoes. And avoid overheating, which leads to sweating and can drench a jogger as quickly as rain -- with an equally chilling effect. Carry an extra pair of socks and even an extra T-shirt if you run the risk of getting wet and expect to slow down or stop running on your way home.

* Drink extra fluids to avoid dehydration, particularly when running or skiing long distances. You lose extra body fluids in the exchange of cold, relatively dry inhaled air for warm, steamy exhaled breath.

* Wear layers of light, loose-fitting clothing, especially if you're going to exercise vigorously for long periods. Heavy clothing not only weighs you down, but is also more restrictive.

* Warm up before you stretch, and stretch before you run. The two are not the same. If you stretch before you're warmed up, you risk straining a muscle or tendon. Try not to start cold -- either stretching or running. If it's very cold outside, limber up inside and get moving as soon as you go outdoors.

* Wear bright-colored, reflective clothing, since winter is also the darkest time of year. Avoid running in the street.

* Resist machismo. In blizzard conditions or other extreme weather, defer to common sense. Stay inside.