Black stirrup pants. Rhinestone bracelets. Madonna-esque earrings. Red nail polish. She is the archetypal teen-age girl ready for a trip to the mall.

Monumental calves. Granite thighs. Shoulders that will never need padding. The smile. She is Mary Lou ready for another exhibition of perkiness.

"I'm a pretty happy person," Mary Lou Retton says, managing to smile and bounce her head and flex her legs and fidget with one of her gold rings at the same time. It is clearly torture to sit still. "I mean, I don't sleep with a smile on. You should see me in the mornings. I hate mornings! I HATE mornings and we have 7 o'clock workouts!"

She grimaces in a smiling kind of way, lowers her voice and booms, "THE OTHER SIDE OF MARY LOU!" Bela Karolyi, her coach, and the beaming woman escorting her laugh at the ludicrous thought. Mary Lou Retton, it seems, has no other side.

In the 15 months since this country convulsed in spasms of affection for the first American woman to win an Olympic gold medal in gymnastics, Mary Lou Retton has become a marketer's dream, one of the country's shortest, most productive cash registers, a burgeoning industry packed into a 4-foot-9, 92-pound package. She's on television crunching Wheaties (warning, "Watch out, big boys!") and selling Ever-Ready batteries and McDonald's and Vidal Sassoon. She's urging "kids" to "Do like Mary Lou. Go for 10" on the bowling lanes of Gold Pin Fun Centers. She joined Ronald Reagan at a preelection Republican fundraiser ("I try to stay away from them," she says of politics) and hurled her way across the stage of the Kennedy Center and put all her money in a trust fund to maintain her amateur standing ("All of it, ALL OF IT! I'm as poor as I ever was!") and appeared at charity balls and wrote a book with Karolyi and the help of a sports writer and gave IBM employes a series of talks she describes as "very, very inspirational."

But the girl-next-door image and the personality that gave new meaning to the concept of bouncy remains intact. Even the press release circulated by McGraw-Hill for her book tour says things like "her mother, Lois Retton, cooks the best Italian food in West Virginia!"

Whither Mary Lou goest, exclamation points follow.

"The fan mail!" she says from deep within an oversized chair at the WDVM-TV studios, where she and Karolyi have just done the Carol Randolph show on behalf of "Mary Lou: Creating an Olympic Champion," the joint biographies of gymnast and coach.

"Fan mail all the time! A lot of proms. 'Come to my prom!' I got some marriage proposals," she says, her face stretching into that familiar smile and her body twitching with laughter. "I said, 'I have to think about it.' "

And the enthusiasm's increasing. Crowds mob her at bookstore appearances, preteens in the audience at WDVM leave squealing, "She talked to me! She talked to me!"

"If you keep your name out there," she says, "people remember you, and that's what I'm trying to do."

Which isn't a bad idea. "I'm only 17, I'm not over the hill yet!" she protests when asked about retirement, but gymnastics is not forever, and when the half-twists and handstands grow shaky, it's nice to know your familiar smile still can sell cereal.

But for now, Mary Lou is continuing her training with four or five hours a day of work at Karolyi's gym in Houston, where she lives with her 24-year-old brother, but she can always make time for things like a diabetes fundraiser in Denver or a White House state dinner.

The '88 Olympics in Seoul beckon, but, she says, "that's really far away yet," a nonanswer delivered as smoothly as any politician's. There's school too, through a correspondence program she began in 1983 to leave her more time to prepare for the Olympics.

"I'm a senior," she says. "Class of '86!"

Her escort, Kathy Gathro, pipes in, "Go for it!" suggesting that the form of enthusiasm known as Rettonism is contagious.

"I mean, I would love to go to college," Mary Lou says. "I don't know if my schedule's going to allow me to. I'd like to go to college -- it's an experience everyone should go through -- but . . . some day -- I mean, not right now -- I want to get married and have kids. Maybe some commentating -- I'm pretty comfortable in front of a camera. I was always kind of an outgoing, straightforward person."

Some gymnasts go on to judge events, but Karolyi says of Mary Lou, "I don't think she's going to have the patience."

"I'm too hyper," she says. "I'd be out on the floor saying, 'You don't do it like that. Ya want me to show you?' "

When Karolyi, the Romanian who guided Nadia Comaneci to the Olympics before defecting to the United States, talks about Mary Lou and the book, it is clear that a sort of Philosophy of Mary Lou has developed, with Karolyi as its prime expounder.

For example, there's the Gymnastic Evolutionary Scale.

"They were three different eras of times," says Karolyi. "Olga Korbut came into the gymnastic scene giving more excitement and more personality into the gymnast's performance, but her technical level was lower, like kids 4 or 5 are performing. Later came Nadia. She represented a physical trend, difficult skills and coming up with perfect executions and technical performances, which was in the time considered sensational -- that's why she was awarded so many 10s.

"Now, since Nadia's period of '76 and 1980, we step into a totally new period with very powerful athletes, physically and mentally perfect developed and prepared who can perform unbelievable hard skills. Mary Lou's performances were not just attractive as presentation, not just for her personal style, but the skills were unbelievable high difficulty. Her performances were sometimes in order of the dreams probably people would think absurd even to think about in 1976."

"Yeah," says Mary Lou.

And like the politicians and corporations who adopted her as a pint-size symbol of everything that is right with America, Karolyi sees a moral in Mary Lou.

"There used to be no one in America telling the students, 'YOU CAN DO IT! YOU CAN MAKE IT!' " he says, leaning forward, his voice rising and fist shaking as if Mary Lou is in a hall about to perform her gold-medal full-twisting layout Tsukahara vault rather than in a chair about to untie and retie her gym shoes. "Now, these teen-agers, these youngsters, they can see how successful somebody can be after, not just before, and not just the pleasure and the unbelievable success of the athletic victory.

"Americans are commercialized, you know, and everyone would like to see her face on TV, everyone would like to see herself on commercials. Now they can see through hard work, through dedication, what you can get and what you can get out of it. That is probably the most important part of the whole Mary Lou story and the whole Mary Lou existence. That's the most important social aspect."

Mary Lou Retton, her existence defined, stretches her legs, cracks her knuckles and smiles.