BYRON JANIS is enraptured. He stands next to the baby piano, literally sputtering his excitement.

From all around him sunlight pours in illuminating the weightless walls of white marble, the graceful leaves of the ficus trees, the imposing Alexander Calder mobile.

"You know what I think?" he asks rhetorically. "I think this building is a modern day Parthenon. And probably," he adds, "more versatile."

The slim, aging Chinese man beside him beams, then practically gurgles with delight, making no effort to conceal his pride.

For I.M. Pei has been working for 10 years on the new addition to the National Gallery of Art. And he is enjoying every minute of the long-awaited adulation.

"It floats, it's joyous and acoustically it's extraordinary," Janis continues, Pei nodding at his side.

"I would call this building the perfect example of 'rubato,'" he went on. "It's a musical term for give and take. If you have one phrase with 1-2-3-4 and you do 1-2 quickly and 3-4 slowly but have them end in the same time, that is 'rubato.'"

"It means," Pei interjects, "freedom within a discipline." He is now enthusiastically joining the dialogue. "Freedom and discipline have to be in balance. Too much discipline and you have fascistic architecture; too much freedom and it becomes Las Vegas."

"Yes, yes, in life too," says Janis eagerly.

"In life, that's right," says Pei.

"And it can't be too systemic," says Janis.

"It must be sympathetic," says Pei.

"That's true of all art forms," jumps in Janis.

"And life," winds up Pei, coming around full circle. Then, "Ah, but I can't even design my own life. That's much too difficult. There," he says pointing to his wife, "is my designer."

End of rubato.

I.M. Pei bursts with energy, with vitality reverberating his presence wherever he is. As the music carries throughout the new building, so do the the explosive vibrations, the effusiveness, the flamboyance of its architect.

Outwardly he belies the cliche of the contained, quiet, inscrutable Chinese.

A Chinese firecracker is more like it.

Yet the serenity, the simple complexity, of the new East Building belie his personality.It is clear that there is some influence from his childhood days in China.

"Well," he says pondering this for a moment as he perches on the edge of a marble slab in the center of the new gallery. "I came here when I was 17 so in a way I was already formed. Yet my architectural training was all here so it was western." He brightens. "But I don't have any schizophrenic feelings about it. If my Chinese background shows up here and there, that's well and good. I wouldn't be surprised.

"I do have a strong affinity for nature," he says, still pondering this question. "I must have brought some of that with me. I spent months looking for these ficus trees. They're ficus nedita. A very special kind."

He stops to think some more, then looks up somewhat confused, and smiles apologetically.

"You ask me all these questions. I really never reflected or thought about before . . ."

He seems to have difficulty articulating what the building means to him. He struggles.

"This building is not monumental," he says. "I'm pleased with that. Awe inspiring is not a thing I would be comfortable with. The problem is how can you do a building of such a large size without having a static monumental quality. One way to do that is to design into a building a certain amount of visual complexity. This building is asymetrical. There are many surprises here. It is, well, it is . . ." he stops and looks surprised. "It is like a Chinese garden." He shakes his head.

"That was not consciously arrived at.

"Yes, yes," he says, like a Chinese garden." He looks around the hall, to one side a view of the Washington monument, to another the Capitol, to another the old Smithsonian building. "You see," he says, with sudden realization, "by changing the variety the Chinese can create in a very small space, many vistas."

I.M. Pei is 61. His wife, Eileen is 57. They look like a perfectly nice ordinary Chinese couple. They act like a perfectly nice ordinary American couple. He has the slightest trace of a Chinese accent.Hers is totally American. Their families were friends in China, his from Peking, hers from Shanghai. They came from well-to-do families. His father, now 86 and living in New York, was a banker. "My father is a good friend of Joe Alsop," he says. "Joe still talks about the wonderful chef my father used to have," he laughs.

Sitting by the waterfall in the sunlit National Gallery of cafeteria, Pei and his wife talk about their life, interrupted occasionally by a fan or a gallery staffer asking for last-minute instructions, delivering a message.

Upper-class Chinese of Pei's father's generation sent their sons and daughters to the United States to be educated. He went to MIT, then Harvard, she to Wellesley.

"We met for the first time," he says, "in Grand Central Station." It was not, they both agree, love at first sight. Later, when he was 25, and she, 22, they were married.

"At the time," he says, "we weren't planning to stay in the United States. We were going to go back to China." But the war changed all that and Pei decided to stay, switching from engineering at MIT to architecture.

In 1951, Pei received a fellowship from Harvard to study architecture in Europe for a year and he grabbed the opportunity, taking his wife with him for what he calls "one of the most important experiences of my life."

"Instead of visiting cathedrals and palaces," he laughs, "I spent most of my time drinking my way through the Rhone Valley."

In Europe, Pei, already a connoisseur of good wines and good food, having trained at his father's knee, really became an expert.

"We spent all our fellowship money on food and wine," he says.

"We stayed in the worst hotels and ate in the best restaurants," says his wife.

"I was already interested in food and wine," says Pei. "But not Chinese wine. I never approved of Chinese wine. And that trip was wonderful for that. But more, that European trip opened up my eyes about western architecture. To go to Chartres, to walk through that building (the cathedral). I consider that a very important experience in my life. If it had happened later I would have already been too formed. If it had happened earlier I wouldn't have gotten as much out of it.The timing was perfect."

He lights up while he talks, leaning forward, confiding, laughing, smiling, rolling his eyes, gesturing with his hands, never ceasing his motion for a second.

During this explosion of thoughts his wife sits patiently by, smiling.

She is renowned for her cooking but modestly declines to discuss it.

"Mostly," says his wife, "I've learned how to be a short-order cook because nobody ever knows the time they're going to eat."

"We eat Chinese food at home a lot," says Pei.

"Eating and art, eating and art, that's what he's interested in," says Eileen Pei.

"That shows you where my interests lie," he smiles sheepishly.

Is I.M. Pei a happy man? "I think I am," he says, slightly discomfited by the question. "Of course, I have a happy family life, number one. I have to say that in front of my wife."

"No you don't have to say it," she says, bristling slightly.

Certainly he is enthusiastic when he talks of architecture. But there is a special look on his face, almost of rapture, when he talks about food. The conversation wanders - back to France, the restaurants, the chefs. He is disturbed.

"Paris is no longer like the old Paris," he says dejectedly. "Restaurants move and change now. A few years ago they never moved. Take Pot au Feu, for instance," he says, getting really serious now. "It used to be wonderful. Then it became so popular and the chef finally quit and moved to the South of France. And I don't like the three star restaurants anyway. They're too difficult to get into and it's too much of a hassle."

He sighs. "I suppose," he says finally, "I'll just have to make new discoveries, new relationships."

"We have four children," he says, returning to the subject. "I am a family person. The family is complementary. My wife's activity makes it possible for me to do what I do."

"You mean," she clarifies, "I keep the family on an even keel."

Her husband is, she says, very difficult to live with, particularly during the conception of a new project.

"He's really impossible," she says matter-of-factly. "He doesn't hear anything. I can't get through to him. You have to write messages out to him, he gets up in the middle of the night. One Christmas I gave him a pen with a light on it so he could write without turning on the light. But it ran out of batteries . . ."

It is necessary," he jumps in, "at the conceptual stage of a project that you must work alone.

"One simply cannot rest until a solution is found. That is the inventive period that one has to go through. It doesn't last long but it can be very frustrating.

"I'm a fairly disorganized person," says Pei. "Don't you think?" he asks his wife. "I'm always making last-minute decisions and changes. On the other hand, you have to have some discipline."

"I think," she says, "you're very disciplined at your work. It's just that in other things . . . like you have no sense of time."

"Well, you know how it is," he laughs, "when you get carried away."

According to both of them, I.M. as everyone calls him, gets carried away a lot. "Many things excite me," he says. "I love the visual arts, I love beautiful things and I enjoy people. There is nothing I prefer than to have a good conversation. I could go on all night. That kind of give and take I had with Byron Janis in conversation, that is my greatest enjoyment."

"Still," she says, "I don't think we're that social. Occasionally you meet someone and it just clicks. We met Byron and Maria (his wife) at a very large cocktail party, for example. They were looking for somebody to talk to and so were we. We just hit it off."

"Eileen and I have lots of artist friends," says Pei, "people like Jean Dubuffet, and Henry Moore, Pierre Schneider. We know lots of writers. These are people we naturally gravitate to, writers and artists. We have very few banker friends. Even though," he chuckles, "my father was a banker."

The Pei's don't entertain that often, they say, and mostly they enjoy staying at home.

Though Pei's dressed in a western business suit he says that he dresses in Chinese robes when he is at home. "I wear a padded dressing gown," he says. "It's very warm in winter and much freer than western clothes. Western clothes are too tight."

His wife, who is dressed in a Chinese-style linen dress, says she wears Chinese clothes all the time.

"I just feel more comfortable in clothes that belong to me and me to them."

"It's not true of our daughter, though," says Pei. "She wears Guccis like everybody else. She says wearing Chinese clothes is like wearing a strait-jacket."

The Peis live in an old brownstone on Sutton Place with a view of the East River and a garden. They bought it and he redesigned it five years ago. They also built a house in the country 30 years ago, which they still love and still use.

"Our house in the city is a pretty special place," says Pei. "We live with a lot of art and for that reason it has to be simple," he says. "But still it is very comfortable, too. Lots of plump soft sofas and comfortable chairs.

He says that his favorite thing to design is houses but that because of his heavy schedule he is not able to.

"Private homes are the most fun," he says. "But I can't do that. I have an organization to worry about. And building buildings like this is something not many companies can do. I can't just say to everybody, now I'm going to take off and go build a little house. Goodbye."

As an architect, he says, one does not have total freedom, not only in the choice of things to do but in the designs themselves. "It is not an individual act, architecture, he says. "You have to consider your client. Only out of that can you produce great architecture. You can't work in the abstract. For instance, if we didn't have the Mellons and Carter Brown, this building would be very different."

It is not, however, his inability to work alone or his lack of time to build small houses that frustrates I.M. Pei.

And he is frustrated.

"I still have never-had a chance," he says, "to design an ocean liner. That is my biggest frustration. Oh," he says with excitement, "that I think would be the most exciting mission I could ever think of. It's a city unto itself. It is self-contained."

"It is," he says, "it is microcosm of life."