Usually, Donna Farina winda up her circus trapeze act by doing the Iron Jaw - hanging by her teeth. But one recent Sunday she skipped that part because "her mother was in the audience and Donna had just had all this orthodontistry done . . ."

The Big Apple Circus is a peoplesized circus. Under a municipal green tent on the 8th Avenue site of the old Madison Square Garden, it sports one (1) ring about 30 feet across, a red plush entrance curtain, canvas murals by Mimi Grooms, a five-man band with a drummer who takes his shirt of when it gets hot, and a cast of several.

There is a Bag Lady clown, a dog act, a Killer Mole, girl roustabouts, a ringmaster who likes to sit in the audience and let small children announce the next act, a contortionist who climbs through a barrell about as big as a hat, plus: jugglers, unicyclists, acrobats, wirewalkers, clowns and - climbing their swaying ladders sideways in traditional style and sliding down the exit rope with legs straight out, toes pointed elegantly - the artistiocrats, the aerialists, the fliers.

Mot of all there are Nina Krasavina and Gregory Fedin, the famous clown-acrobats who left the Moscow State Circus four vears ago to perform and teach in America. They are the muscle and brain of the New York Scool for Circus Acts, and nearly half the acts in the Big Apple Circus are their students.

"We love to work with the young people," says Nina. "They burn!"

She and her husband are both short and stocky but not in the least bit roly-poly, which is how they appear in costume. They sit in their unclutered apartment, both talking at the same time. Their English is articulate and imaginative. In the background, two cats attack a piece of steel-pipe rigging apparatus that lies on the floor.

"A lot of kids come to us, and they want to be someone but they don't know who, they want to be special. But they don't realize the danger, the hard work."

Gregory is speaking now, jumping to his feet, eyes alight, hands slashing the air. "The first thing, the very first thing you have to learn is discipline. How to organize your body, your nerves, your muscles. Total concentration. Total observation of your own body. After that, they can do anything they like."

He rages when students drift into rehearsals carrying coffee cups and newspapers. "They should be concentrating, thinking of what they will be doing. And how they'll look: It's show business, after all."

The first two things he was taught in the circus were how to splice a cable and how to take a bow.

His present act requires him to monkey-walk up a pole on hands and feet, then hang out over space while his wife swinging on it. Then he spins her.

On fear:

"I've been doing it 22 years, and every day I'm scared. Every day I fight my nerves. Every day I win."

Mia Wolff, blond and 26, has rope burns on her forearms, behind her knees and all across her back. "Those are my mistakes," she says. She is the catcher in a double trapeze act with Donna Farina.

She was an art student at Pratt Institute, left there to paint but wanted something more and became a truck driver. Latter she drifted to mime theater. About two years ago she ran into Nina and Gregory and got a load into Nina and Gregory and got a load of those spangled people flashing through the air, and then she knew who she wanted to be.

The act is scary to watch, as Donna pirouettes upside down on a rig held by Mia, who herself is dangling by one foot. Or when they do an eerie ballet from their swings at the very top of the tent.

"When you get a rope burn you can't just say, 'Ow, I gotta stop.' You have 230 pounds of people depending on you. We make our own costumes, and once, when we were using this stretch fabric, it got twisted around the bar and we were stuck together in midair - in front of an audience - and Nina had to climb up with a knife and cut us free."

She likes practicing in the Synod House at St. John the Divine, where she sit on the trapeze and look through the stained-glass windows. "But the tent's better, even though it's hot up there. Your head is hotter than your toes."

The idea belongs to Paul Binder, 35, the ringmaster. As a comedy juggler he roamed Europe and the Near East for years, winding up at the Ecole Nationale de Cirque in Paris. Five years ago he was discovered by Annie Fratolini, of the venerable circus family, and moved to the Nouveau Cirque de Paris, when he fell in love with small circuses.

"You sweat with the performers, you miss with performers, you make it with the performers. The ring is a hub of energy, and the performer conducts the energy to the audience. What makes a circus special is its reality. There are no illusions: That girl really is up there on that wire. That's why the closeness helps. You see it's a real person doing it."

When he was trying to decide whether to stay in Paris or return home, he asked his cat. The cat smiled. So he came to New York.

"We dramatize everything, of course, because circus is theater. But it's a reality dose. Reality is magical, and if we've touched 'em in their hearts they'll see magic everywhere."

In its first season last summer, the Big Apple drew 45,000 people to its Battery Park site at the foot of Manhattan. This year they were to have Bryant Park, but instead wound up on 8th at 50th Street, a poor location which Binder feels is slowly killing them. Private and government grants have helped, but there is no substitute for audiences. With a payroll of 53 and a break-even point of $17,000 a week, he's concerned about even holding out until the Sept. 10 season closing.

"We don't want to lose the institution even if the circus fails. We may take it around to places outside New York. The dream is to have a permanent circus site, the way they do in Russia [where they are 100 permanent sites for the Moscow State Circus plus 200 traveling tent shows, manned by 8,000 performers]."

He is proud of the specially funded inner-city program for 45 youths which has produced a dazzling six-man acrobatic team called the Back Street Flyers, who do what amounts to an aerial act without a trapeze. There is also an after-school gymnastics program aided by the Dance Theater of Harlem.

"I like the evening show," Binder says. "It's quiet, with an adult audience, and there's no vending in the tent. The focus is clearer then. The concentration."

Michael Moschen's juggling act is classic, brilliant. In a big circus it would be lost.

Dressed in black, moving with the spare grace of a dancer, he juggles three white balls so precisely and simply that you forget to breathe.

The balls obey him like live things. He rolls one down his arm to his fingertips. It stops at the edge. Then, slowly, unbelievably, it rolls over the edge - and under. And hangs there.

A ball loops through the air, lands on his arm. His back. Jumps to the top of his curly head. To his forehead. Suddenly he tilts his head, and the ball now rests, without a jiggle, on his temple. Try it sometime.

("I like it when Michael comes to our rehearsals," muses Gregory. "He's an example. He comes in, doesn't say a word, just sets to work. No coffee cups. No nonsense, Eight hours a day.")

Moschen has all the marks of the master performer: economy, mania for perfection, and an inner stillness which hypnotizes an audience into concentrating as totally as he does.

He did the fire-juggling scene in the new film of "Hair" and has appeared on various TV shows including "Sesame Street." He used to juggle for coins in front of the Metropolitan Museum. He will be heard from again.

"When you love something," Nina says, "you want it desperately. You'll do anything."

Raised in Leningrad, she was to be a dancer and went to ballet school for five years but didn't grow tall enough.

She switched to a state gymnastics school and became an acrobatic clown, which she liked better because it brought her on stage more, and for 18 years she starred in the Moscow circus. She has a master's degree from the Leningrad sport school and has performed with the Kirov ballet.

She and her husband left Russia under a Jewish emigration quota four years ago. Gregory, a Muscovite, has a degree in physical education and also a technical education which gave him his metalworking skills. He has built every bit of equipment in the circus, not counting the rigs of visiting artists who appear regularly with the show, and he makes a point of teaching to students how to fix their own gear.

"You can't be helpless if your stuff breaks down and you're out in some town somewhere. We teach them how to do everything."

His wife nodded. "Everything, everything. The costumes. The style. And the work. Some take longer than others. Foot-juggling takes longest, but there are some who learn faster."

Even now, after all these years, the Fedins practice, alone and with students, four to eight hours a day. They are on a permanent diet, take a drink only on days off, smoke a little. The two cats are their children.

How do they know when a student act is ready?

"It's like a baker, You know." She smiled brilliantly. "Then the day comes when they climb up there to the top of the tent and you can see in their faces: 'Look! I'm not a waitress anymore!' . . ."