When Judi Sheppard Missett created Jazzercise in 1972, sho hoped it would become "the McDonald's of exercise."

"I wanted," says the former professional dancer, "to make the joy of dancing and the exhilaration of a fit body available to as many as possible as cheaply as possible."

Nine years later, she's come close to realizing her dream. The California-based dance-fitness program has 150,000 students and 1,200 instructors in 42 states and eight foreign countries.

"And we're still growing," says Jazzercise administrator Cindy Alexander, "if you'll excuss the expression -- by leaps and bounds."

Jazzercise is just one success story in the flourishing field of "dancersize" -- a catch-all term for the scores of programs popping up around the country that combine the conditioning benefits of aerobics with the flexibility, fitness and fun of dance.

About a dozen variations on the dancersize theme are offered in the Washington area -- in recreation centers, churches, schools and workplaces. Thousands of people, most of them women, from teens to senior citizens, are dancing their way to fitness.

"They're our hottest classes," says Linda Norton, exercise and fitness supervisor for the Montgomery County Recreation Department, which offers four different varieties of dancersize. "They've been growing for the past three years now, and it hasn't seemed to peak yet."

"They're number one here," adds Bill Yeargo of the Fairfax County Recreational Department, which runs 2,000 students through its Aerobics class each year. "Most of the people who sign up for one session keep coming back."

The trend started, most fitness experts agree, with Jacki Sorensen's Aerobic Dancing, Inc., a 10-year-old program created by the 38-year-old California choregrapher and former professional dancer.

Sorensen capitalized on both "the fitness boom" and "the dance craze" by incorporating the principles of aerobic conditioning into a series of dance routines, choreographed to popular music. The movements are relatively simple and contain enough repetition so nearly anyone of any age can follow the instructor and dance along, wven their first time out.

Aerobic Dancing, Inc. now employs 3,500 instructors (who must be nonsmokers and pass a fitness test) and has 135,000 students in 40 states, Japan and Australia. Some of the varied "spinoff" programs cropping up throughout the country were started by former Sorensen instructors.

While each dancersize program may vary according to the choreographer's style and the instructor's personality, most of the good programs are similar. (For an exercise program to be considered "aerobic" -- which means with oxygen -- it should make the body work at peak capacity for 12 to 20 minutes, at least three times a week.)

Classes are usally 45- to 60-minutes, inlcuding warm-up, high activity and cool-down periods. Routines are choreographed to improve flexibility and muscle tone, trim the body and strengthen the cardiovascular system.

In some programs, students repeatedly take their own pulse to judge the effect on their heart. In a few, students go barefoot, while most require tennis or running shoes.

Jazzercise adds "a primal scream aspect" to its program. Says Virginia Jazzercise coordinator Chelle Tierney, "We encourage students to whoop and holler as they dance. It's a great way to get rid of tensions and feel like a kid again -- which is terrific if you've just surpressed screaming in a slow-moving supermarket line."

Like converts to any movement, dancersize devotees often become missionaries for the program they attend and loyally claim that their teacher is "the best." f

"Many of these people get a strong allegiance to a specific instructor," says recreation specialist Norton. "It's like a fever. Our registration usually starts at 7 p.m., and people begin lining up by 2 in the afternoon. If they register late and the class is closed they practically cry."

Why do people love it so much?

"It's a form of group therapy," says Manassas dentist Jack Payne, 50, a participant in Trim -- one of the few programs that actively recuits men. "The first night out I felt like a klutz, but everyone laughs together -- and sweats together -- and has lots of fun."

"Misery loves company," says Fairfax recreation specialist Cheryl White, who takes Jazzercize. "It's fun, but it's also hard work. For a lot of people it's easier to go to a group class then to go out and jog alone. Your muscles hurt, but your neighbors muscles hurt, too.

"You may go in there dragging, but you come out with lots of energy. You really feel good, look better, sleep better. And it's cheaper than a psychologist."

"It's fitness without drudgery," says Marty Trbovich, Washington area coordinator for Aerobic Dancing, Inc., which has 7,000 students locally. "Even if you're not a fantastic dancer -- and 90 percent of us aren't -- you can get out there and imagine you're Cyd Charisse."

"It's non-competitive," notes Toni Cookson, a 39-year-old former elementary school teacher who created Dance Dynamics, Inc. "You may get off a tennis court more frustrated than when you got on, but here you can relax, act silly and cut loose."

For interested dancersizers, Amy Jones, women's program director of Dr. Kenneth Cooper's Aerobic Center in Texas, his this advice:

Listen to your body. "Don't overdo it, particularly at first, or you may be setting yourself up for injury."

Warm up well. Particularly be sure to stretch out your calves and Achilles tendons.

Consult a physician, before starting dancersize or any other fitness program. Persons over 35 are advised to have a stress test.

Try a slow-paced exercise class, first, if you're extremely out-of-condition. You may need to work your way up to dancersize.