It was 6:15 p.m. The temperature was 93 degrees.

In a corner of a large, unairconditioned gym, 10 youngsters ignored the sticky heat and lined up at the uneven parallel bars to swing through the routines.

Along one wall, a dozen girls took turns walking, leaping, turning and posing on a four-inch-wide balance beam.

From another corner, one gymnast after another raced across the room to vault over a side horse, while another line of girls practiced their tumbling.

"New trick," someone called. All activity ceased. Tammy Smith, a small, dark-haired girl in a green leotard, ran forward. She sprang into a complicated cartwheel-twist, a flip-flip and a triple twist. It was a "triple full," an extremely difficult six-second wonder of movement.

Tammy, 13, is one of more than 50 gymnasts working under the guidance of Margie and Greg Weiss, owners and directors of MG Gymnastics of Silver Spring.

In the 10 years the gym has been open, the Weisses have fielded several champions -- including their first student, Leslie Wolfsberger, a member of the 1976 Olympic team, and Jackie Cassello, a gold medalist at the Pan American Games and a member of the 1980 Olympic team.

The Weisses' line of winners continues. Tammy Smith, classed as an advanced gymnast, will enter "elite" trials, the next class up from advanced, next January.

Margie Weiss concedes that attaining advanced and elite ratings requires great sacrifices, but she says the champions meet that challenge. "These are a driven, determined type of kids," she said.

Part of what is sacrificed is free time. Tammy Smith, who lives in Westminster, commutes three hours daily, six days a week, to work out six hours a day at MG Gymnastics. For the last five years, she has had little time for anything but gymnastics.

Another gymnast, Kathy Fineran, 16, sees symnastics as a way to get a college scholarship -- as well as to have fun and travel. Her most memorable experience came in 1977 when her team competed in two meets in Germany.

The gymnasts concede that the need for discipline poses some drawbacks.

"Sometimes you can't go out at night -- you have to work out," Kathy said. "Or you might have to miss a Saturday football game." She added, however, "Sometimes the older girls get Friday nights off."

Several of the Weisses' students who come from areas outside Washington board with the Weisses or with other families while they are training.

One of these boarders is 10-year-old Nancy Lemenager of Massachusetts, who has been with MG since early last month.

"Nancy is something super," says Margie Weiss. "In her first two weeks here she learned five tough skills on the (parellel bars."

Nancy came to MG after her Massachusetts coach decided she needed more advanced training. Weiss says Nancy got "excellent coaching in the basics in Massachusetts."

Nancy has her eye on the 1984 Olympics, and the long hours of training and living so far from home don't bother her at all.

"I like doing what I do," she said. "I like to work and be competitive."

It's not that simple for her mother, who, like many parents of gymnasts, acknowledges the sacrifices involved.

Whole families have to schedule their activities around gymnastics workouts and meets. Traveling costs can be high.

"There's no time for friends in school," said one mother. "They have to buckle down and budget their time."

But Weiss explained, "The parents realize their kids are pretty driven. They don't want to deprive them of their chance. They realize it's now or never."

Maureen LaGrua, 11, is an advanced gymnast from New Jersey who has been on the MG team for a year. She moved in with the Weisses last month. Her mother has mixed feelings about the arrangement.

"I think Maureen's having more fun than most kids her age," said Elizabeth LaGrua. But she added, "Every couple of months I see her. I really miss her."

LaGrua, whose older daughter also is a gymnast, said Maureen is "very independent. One day I told Maureen that if she kept on working, someday she'd be as good as her sister."

Maureen's reply at the time was, "I'm not going to be as good as Tricia -- I'm going to be better." She was then 4 years old.

"Gymnastics has a fairy-tale type of appeal, especially since Olga Korbut," said Margie Weiss. "It's got an image of 'petite,' 'elegant,' 'dignified' -- not sweaty.'

"Also, it's a varied sport, not like piano lessons. If they get bored with one thing, they can try something else, work on a skill, practice a routine.

"Flying through the air has always had a basic appeal, which they are doing 90 percent of the time."

Weiss says schoolwork rarely suffers among serious gymnasts.

"Greg says there is no such things as a dumb good gymnast," she said. "There are so many variations to learn and remember. A disciplined gymnast tends to be a disciplined student."

All told, the Weisses believe they have about 12 "top level girls coming in this year -- a whole new crop of athletes."

The MG staff of nine, including the Weisses, coaches, a ballet teacher and a choreographer, concentrate on "top-level coaching" with a strong emphasis on safety.

"I enjoy working with these exceptional kids," Margie Weiss said. "You can see so many accomplishments in so many ways.

"All our kids are trained toward what we think is their potential. They feel good about themselves."