Marta Istomin's office is the largest in the Kennedy Center. A bank of tall windows faces east, through which the morning sun burnishes the walnut paneling. There is a black Steinway in the room, and a single massive desk.

The office was Martin Feinstein's when he was executive director for performing arts of the Kennedy Center, but Feinstein is gone now.

In his chair sits a small woman, undeniably attractive, her raven hair swept back, the bright blue of her dress a startling antithesis of pin stripe.

Less than a month ago, Marta Istomin, 43, was named to the newly created job of artistic director at the Center -- an appointment that came as a surprise. Her duties were sketchily defined as a coordinator of functions and factions, and a Center spokesman seemed at pains to establish what she was not: "She's not going to be a replacement for Martin Feinstein."

If Washington is curious about her, however, she returns it in kind, and without ambiguity.

"I like to listen, and to ask questions, too," she said, coming out from behind the big desk. "So I'm not surprised that people wonder what my role will be here. My rank, as you put it." She smiled and tossed her head, the china cup in her right hand maintaining its equilibrium as if on gyroscope. "I'm supposed to be No. 2 at the Kennedy Center. And I intend to be No. 2."

"Certainly," said Roger L. Stevens, chairman of the Kennedy Center. "We don't go for red tape around here," he added in his characteristic gruff style. "Martha is No. 2, all right."

So as of Feb. 26, Marta Istomin took office as the highest-placed performing arts administrator in Washington. To most people, she was twice a famous wife. She was the beautiful cello pupil who married the legendary Pablo Casals when he was 80, and she 20. And she is presently married to Eugene Istomin, the distinguished American concert pianist.

Marta Istomin was chosen as artistic director, Stevens said, "not by any search committee. That's not the way we do business. Abe Fortas brought her name up. He and Marta go back a long way."

Former Supreme Court justice Abe Fortas, a member of the Kennedy Center board, chose the word "instrumental" to describe his role in bringing Marta Istomin to Washington.

"I was a director of the Casals Festival in Puerto Rico, and I got to watch her in action back then. Her operational abilities are superb, and she has great experience in dealing with conflicting personalities. When things get confusing, she's extraordinary. She'll be a tremendous, seminal force at the Center, but it will take some time.

"Don't even forget one thing about Marta," Fortas added. "She's been tempered by fire." No Dragon Lady

Washington is just getting to know Marta Istomin, but Eugene Istomin has a head start. They've been married since 1974, and he was a friend and protege of Casals before she was.

At the moment, Eugene and Marta were posing for pictures on the balcony of their Watergate Hotel suite. A brisk wind confounded the scene.

"I'm a tropical bird, I won't last long out here," she exclaimed happily, throwing her arms around her husband, an impeccably dressed man of 54 whose aristocratic air shattered instantly like a wine glass exposed to Memorex.

Istomin's eyes followed his wife as she fluttered about for a few more moments, and then darted out the door and back to her office at the Center nearby. He is frank in his belief that she will shortly be a force to be reckoned with in the arts.

"Surely Marta has ambition, yes," he said, "in the sense that this appointment means a great deal for Latin American women. Don't worry, she has no intention of becoming Evita Martita, or the Dragon Lady of the Kennedy Center, or anything like that.

"But she is tremendously independent. I've never seen her adopt a popular view if it goes against her ideals, and she has many ideals -- as a Latin American, as a woman, as an artist, as a person. The job here will be a challenge, because you don't succeed in public life without major compromises.

"She can remain independent, because she has a very substantial income already, and so do I. We're both making sacrifices so she can do this, and the greatest is that we'll see each other less. I'm going to pass up long concert tours from now on, but even so we'll be apart. And we'll have to ride the New York-Washington shuttle more, and how I hate that shuttle." t

It was 7 o'clock on a Friday night, and the pianist was about to resume his daily practice. First, he pondered Fortas' phrase for a moment.

"'Tempered by fire . . .' I'm not sure what he meant by that," Istomin said. "Maybe he meant that Marta has been married to two great artists, and that's not easy, you know. Casals could be very difficult. And I am not modest -- I can be difficult, too."

Istomin smiled. "In the past 60 days I have made 50 trips to perform, and the performances don't get easier every year, they get harder. An artist reaches further every time, he takes risks, and it's difficult for those around him. Marta understands." Casals' Secretariat

She also understands why her marriage to Casals follows her even now, a legend in rondo form, its refrain recurring in different keys. In 1956, Casals was in Prades, his tiny Catalan village in France, having vowed never to return to Franco's Spain. He was joined there by Marta Montanez, a well-born young cello student from Puerto Rico. It was she who persuaded him to return with her to Puerto Rico, and found the festival there.

"I was the emissary," she says. "The government wanted him to come, and he liked me and he liked Puerto Rico -- his mother had been born there. It wasn't hard to persuade him."

They were married in Puerto Rico, and the marriage was not taken lightly. "I listened to people's advice," she says. "That was a case in which I did not take it."

Marta Montanez was a devoutly religious young woman, and Casals had been married and divorced twice before. There was also the 60-year disparity in age. Spanish custom was strict, and they were not excepted.

As Casals himself, on the occasion of his 87th birthday, explained to Washington Post music editor Paul Hume:

"I had been sick, and she nursed me back to health. She remained in my house overnight. It is the custom when a woman stays in the house of a man, whatever the circumstances, that they be married. I went to her father and asked for his permission, and he gave it."

So Marta Montanez Casals plunged immediately into the maelstrom of Casals' political life. He was a man of unyielding principle, and he had already announced that he would never play in the Soviet Union, or Germany or Italy, nor return to Franco's Spain. Every performance he gave had political meaning.

"Oh, it was so complicated," Marta Istomin said, her voice rising. "I became a one-person secretariat. The phone rang all the time, and I answered it every time. I handled all the correspondence, all the demands, and saw all the arguments.Casals was the sort of person who had to decide everything himself, and he accepted invitations to perform only when they furthered his political views.

"That's why we came to the White House at President's Kennedy's invitation, to play that famous recital. Maestro Casals didn't agree with the United States policy regarding the Spanish refugees, and he wanted to get the president's ear. And believe me, he did."

Martita, the diminutive nickname by which she was known, also served another way. She was the only one who dared suggest to Casals that he might not have performed up to his own difficult standards.

"Well, yes, he would ask me what I thought. I would tell him. After all, I was his student, and I certainly knew his view of art. I would say, it went this way and that. Many times he would not agree, however, and we would . . . discuss it. There was never any question on correcting him, of course. But I would say what I thought right out. I hope I still do today."

Istomin remembers those days, too. He was with the master long before his move to Puerto Rico.

"It was a game for us then," he said."Casals loved to have young people around, and he used to make a great show of not practicing, and we would urge him and prod him. Because he suffered, yes, all his life as an artist. The famous story is true -- a rock fell on his hand once in the mountains, and he thought the bones were broken, and his first reaction was, 'I am free of the cello, finally!'"

"Those were difficult years," says Marta Istomin. "There were many sacrifices, and I learned a great deal during our 16 years of marriage. I speak five languages -- English, Spanish, Catalan, French, Italian -- and I learned several of them just doing the Maestro's correspondence.

"That must be what Mr. Fortas means when he says "tempered by fire.'" Hard-Headed

The legend of the Casals' years overshadows her other careers, but the fact is that Marta Istomin is an administrator and a businesswoman of some experience.

She is on the board of Harcourt Brace Jovanovich, the New York publisher. She is on the board of Sea World, an HBJ subsidiary which operates marine environments in Orlando, Cleveland and San Diego (she says "Than Diego," a hint of Casals' Spain in otherwise unaccented speech). She is a director of Marymount College in New York, her alma mater, and of the World University of Puerto Rico. She single-handedly maintains and guards the Casals Archives, which are stored in a room at New York's Mayflower Hotel, where she and Eugene maintain a residence. And she was, until three years ago, music director of the Casals Festival in Puerto Rico.

"I asked her to become a director in 1977," said William Jovanovich, the crusty president of HBJ. "The next year she also became a consultant for us on educational work for Latin America. I'll tell you this about Marta Istomin -- she doesn't waste time, she doesn't quibble, and she has experience with cultural politics, which are the most vicious of all. And she's willing to go out on a limb." Jovanovich has seen it.

"HBJ is the only Fortune 500 company which presently forecasts earnings for the future year," he said with some pride. We're all encouraged to do it, but none of the other companies want to and you can see why.

"Well, our directors got to arguing, and we finally came up with a projected share earning of $6.15. So one director says, 'Listen, to protect ourselves we should announce $6 even.'

"But Marta spoke right up, 'I disagree,' she said. 'We pay our managers well, and they should be pressed to do their best.'

"That's pretty hard-headed," Jovanovich said. "Her comment turned the tide, and we went with the $6.15."

Marta Istomin was equally hard-pressed about the Casals Festival when several years ago it began to stray from its initial course, which was to present the music of Casals' taste -- chamber music, string music, not much opera, very little modern music. The government cut the budget, and instituted programming measures designed to gain bigger audiences. So Marta Istomin changed course, too. She quit.

"I wish them all the best," she said. "It's just that I can no longer be a part of it in name. I have removed myself, and now they will go along without me." Flowers and Force

Martin Feinstein, in whose office Marta Istomin now resides, has not gone far. He is now the director of both the National Symphony and the Washington Opera. While Mrs. Istomin was observing the world through Casals' lens, Feinstein was learning about it from the master impresario Sol Hurok, in New York.

They know some of the same people, but they do not know each other well.

"It's not really clear to me what Marta will be doing at the Center," Feinstein said. "I've known her for a long time, but not intimately, not, say, the way I've known Isaac Stern. I've seen her at cocktail parties. We chatted before she took the job, and I told her the things I'd started. But of course I had already given a list to Roger Stevens through 1982.

"I also left her two presents. One was a floral arrangement that said 'good luck.' The other was a button I'd had for some time. It said. 'May the Force Be With You.'"

Feinstein was the executive director of performing arts for the Kennedy Center for eight years, during which he made no secret of his ambition to bring the finest European operas and ballet companies to Washington. Much of his ambition was realized, and most everyone realized he was ambitious. When he left the Kennedy Center, his job title was abolished.

Marta Istomin does not have the contacts that Martin Feinstein does. She is not a member of that web of interlocking friendships, experience, history and opinion. In other words, she is not in the arts mafia. Whether that will be useful or detrimental remains to be seen.

"It is true that I am not a member of any . . . what you said," she acknowledged. "I have always been independent, and I have my own feelings and knowledge. I listen to the advice of others, and to their experience, but I always reach my own conclusions."

Like Feinstein before her, Marta Istomin will plan festivals and coordinate bookings. Many of the bookings will be those of the National Symphony and the Washington Opera.

"I expect we'll be working closely together," said Feinstein. Special Ambiance

Already, as the door to her big office opens and shuts and the desk phone rings, the problems begin to flow to Marta Istomin. She is still careful, after only weeks on the job, to avoid forecasting specifics of her contribution in the months to come.

It would seem, however, that she intends to open doors.

"Art is a doorway, and there are still many people who haven't crossed through it. And there is also a doorway for artists, and we must bring performers here who aren't well known yet, but may be very talented. Renown is not necessarily a mark of quality. Roger Stevens wants more American artists, and I agree. There should be no problem there.

"I know that everything is complicated. But I want to create a special ambiance here. I want the busloads of people who come to the Center to hear music, real music. I want performers in the halls, little informal groups that will fill the air with music during the day. The whole building should be alive, and that ambiance we can achieve.

"But I wouldn't want anyone to think that I came here with the idea that we could just let my role evolve gradually, to see how things work out, and so on. No. I have a specific understanding with Roger Stevens."

She does not deny a reputation as a hard bargainer.

"I do defend people's rights, although maybe my own less so, at least financially. But I will defend conditions, and ways which make it possible for me to function. I must have possibilities, in terms of freedom, in physical matters, and of staff. I know how important those things are, from my own experiences. Yes, I have a big office, but I'm going to make it smaller. We need the space for my staff."

"I'll tell you what 'tempered by fire' means if you don't use my name," said one close observer of the Istomin-Washington connection. "The Center came looking for her, not the other way round. And I think she gave them a hell of a ride before she said OK." Under Examination

In fact, Marta Istomin has not yet actually signed her contract.

"Oh come on," said Stevens. "Everything is agreed on in principle, and there's no problem. As far as salary goes, she'll be commensurate with the top people here. Maybe she won't get what Martin got at the end ($72,000 a year), but it'll be the equivalent of a congressman's salary. And there are conditions beyond salary.

"As far as offices go, sure hers is bigger than mine. When I owned the Empire State Building I had the smallest office in it. I never wanted to look out the window anyhow. She's the one who needs the prestige, not me.

"Her title means exactly what it says: artistic director. I'll keep my hand in theater, because that's all I get out of this job anyhow." Stevens, who has served as Center chairman without pay since it opened, has produced more than 200 plays in his career.

"Marta's going to have other duties, too, and she knows it," Stevens said. "I'm tired of haggling over flowers for stars. She can do a lot of stuff better than I can. She's going to have to see the chancellor of Germany or the Japanese premier when they come by, because I'm no good at it and they'd rather see her anyhow."

Stevens, who recently suffered his second heart attack, confirmed that he plans to cut back his own schedule at the Center. "First we're going on a marketing push, but when that's over, I'll just have to slow down. There'll be plenty for Marta to do."

Isaac Stern, the best-known violinist of the times, has known Marta for years, and with her husband is a member of the highly regarded Istomin-Rose-Stern trio, with Leonard Rose. He thinks "tempered by fire" an appropriate phrase.

"Remember," he said, "being around Casals meant living in exile. It meant working with refugees, it meant standing for something all the time.It wasn't all U.N. concerts and trips to the White House.

"And after he died, she carried on his festival with people sniping at her heels, with artistic pettiness all around, and in the midst of Puerto Rican politics.

And it also means personal things, like living with Eugene Istomin through the death of his mother.

"I'll tell you this about Marta: she has exquisite manners, she has taste, and she's not fakir. She's not a gossip, and she's not an intriguer.

"She's taking on a difficult task in an area that is not entirely charted. Part of what she'll be doing was done by Martin Feinstein, and part by others. It's going to be a hell of a job, and she'll be under sharp examination all the time." Sacrafice

Marta Istomin says she is rady: aware that the situation is complicated, confident that her opportunity is real, and willing, as she puts it, "to make the sacrifice."

The house she and Eugene maintain in Puerto Rico will see less of them than ever.

"I've made hard choices before," she said. "Maybe I'm used to it. Anyway, I'm not very good at relaxing. You know I grew up in Puerto Rico, where the beaches are beautiful and people fly in on airplanes just to swim. In my whole life in Puerto Rico I don't think I went to the beach five times."