Devotees call swimming The Perfect Exercise. It conditions the heart, improves circulation and works just about every muscle in the body.

But swimming isn't the only way to get a water workout. Swimnastics -- a program of exercises in the water -- has many of swimming's advantages, yet can be done by people of all ages who want to slim down or tone up a specific area of the body.

And you don't even have to know how to swim.

When you do an exercise in the water, "You get twice the effect," says Ginny Gilstrip, 52, who has taught Swimnastics and Slimnastics for 16 years at the YWCA, 1649 K St. NW.

"Because you're pushing against the resistance of the water, you don't have to do the exercise as many times to gain benefits. It's almost like working with weights. And it's easier to suspend a stretch in the water, which is a beautiful, delicious feeling."

Movement in the water, because of the buoyancy and relative loss of 90 percent of body weight, seems easier than on land. And because most of your weight is supported by water, there's less wear and tear on the joints.

This buoyant support, says Gilstrip, "makes it very hard to hurt yourself in the water. It's impossible to move in a jerky fashion, and you have to move slower."

These advantages make swimnastics particularly well-suited, with a doctor's approval, for older people and for anyone recovering from an injury, says Gilstrap, who teaches a special class for women over 65.

"Sometimes their doctors prescribe it as physical therapy. I've had women come in here who could hardly walk, and after a year of swimnastics not even need a cane."

"I could never imagine myself jogging," says "over-65" class member Leola Stahl of Northwest Washington. "But we jog in here, and it feels just great."

Trained in nearly every form of dance, from classical ballet to belly, Gilstrap incorporates a wide variety of movements into her swimnastics class.

But nearly any exercise, "except maybe sit-ups," she chuckles, can be done in the water and turned into a swimnstic.

And what other outdoor exercise can you do for an hour under a searing summer sun and still remain cool?