Leotard-clad Kathryn Fredgren, dance of "Once in Love With Amy" for her "toe-heel-step-flap-back-step-step" routine around the scuffed parquet floor.

"Come on, everybody nice and light. Gene Kelly style, just like you're walking down the street," Fredgren instructed the tap-dancing class.

And for an hour this week Fredgren's students - all adults - danced their way into another world, just for the fun of it.

They didn't pay particular attention to their own thick thighs, bulging stomachs, weak ankles or shy gazes in the wall of mirrors befor them as they glided soft and swishy to the oldie "Yearning (Just for you)."

And an occasional stumble didn't deter them as they executed the fast syncopated footwork of shuffle hops, flap-turns, stomp-claps and wing-pull-backs to "Ain't Misbehavin." and then paused, sweating, to catch their breath.

"Simle, everyone!" a prancing Fredgren crooned, "Spot where you're going . . . Gooood. That's very nice."

The American art of tap dancing, adored by the fast steppers of the '20s and '30s and the dance school darlings of the '50s, is making a comeback after 20 years - and among adults.

Septuagenarians, college students teachers, homemakers and government workers are among the predominantly female following of a dance form that has become a sport, a lapse into nostalgia, a fun way to exercise.

Twenty-nine years ago when "Mr. Mack" (Mack Krents) opened his Artistic Dance Fashion shop in Bethesda "the biggest thing around was tap. There were huge recitals:" said Marti Chidsey dance supervisor for the Montgomery County Recreation Department.

But parents stopped sending their children to tap schools and many local ones closed their doors. At the turn of the '70s came the revival musicals like "No, No, Nanette" and "That's Entertainment" and the sudden adult interest in tap dancing led to a renewed popularity among children.

The adults still have the edge on the market. "All of us idolize the movies," said Fredgren. "In performing dance, we're touching the star world. I've got a 57-year-old woman in a jazz class. She's never moved (like this) in her life."

Three years ago, Chidsey, offered experimentally an evening tap-dancing class and immediately "it just filled up."

"Then I put in a second one, and it filled," she said. This tall, four teen and adult top classes along with jazz dancing and ballet are taught through the department.

"People are very candid on the phone," she went on. "A woman told me she had just recovered from the trauma of divorce and she thought tap dancing would be jsut the thing for her."

"It's tremendous therapy," said Fredgren, a mother of three who until recently hadn't tapped for 17 years. She conducts a half dozen tap and the equally popular jazz (show) dancing classes at Arlington Dance Theater.

"Everyone needs a release today. It's more fun than doing exercise and you can come in here and forget your own name for an hour," she explained.

"I could not really bring myself to jog," said Kathleen Murray, a middle-aged professional skater. In tap, "I think there is a lot more emphasis on your body."

Lanny Slusher, a 26-year-old pressman who works all night, grabs some breakfast and heads for Fredgren's weekly session, found tap dancing "cheap fun. Sometimes I dance around the house," he said. "Gene Kelly is my favorite dancer and I decided I'd try this."

Christine Chaconas took up tap again last year after her 14-year-old daughter began lessons. "I studied as a kid and the steps came back to me. It's funny how you don't forget them."

Adults are tapping all over the metropolitan area and around the nation at lunch breaks (although they might not admit it), on the wooden rec room floors, in sorority houses, across concrete sidewalks and in mirrored dance studios.

Chaconas said her friends "just laugh" when they learn of her hobby, "but once they see me do it, they want to try it too," she added with a confident grin.

Since 1973, Capezio tap shoes have quadrupled in sales nationally and in dance stores from New Carrollton to Bethesda, merchants are busily placing "telephone," "staccato," "hi-fi" and all manner of taps on women's shoes with two-inch heels and men's on fords.

"It a lively dance form mentally, said Linda Rosenberg, who teaches in College Park. "You have to really be on the ball and coordinate your feet. You're making a noise and you can hear what you're doing. You get the result quickly. It's excitement and fulfillment."

Students pay $2.50 to $5 an hour for classes locally and from $11 to $37.50 for women's shoes and about $22 for men's. Taps are an extra $3 to $8.

Arlington Dance Theater, the Montgomery County Recreation Department and dance studios throughout the area are constantly receiving calls "just begging for more tap classes, according to teachers.The dozens of classes available enroll their quotas quickly.*