Mstislav Rostropovich was sitting in the presidential box of the Kennedy Center Concert Hall with Rosalynn Carter last night, and he began to worry when she left unexpectedly halfway through the concert celebrating Isaac Stern's 60th birthday.

"Near the end of the Barber Violin Concerto, a messenger came in and whispered something to her." the music director of the National Symphony Orchestra recalled, "and she turned to me and said, 'I'm sorry, I must go now.' And I began to think how hard the presidency must be if they can't even take time to enjoy music. Then a few minutes later, I saw why she had to leave, and when I saw her coming on stage in the golf cart, I was surprised but happy."

Abe Fortas, presenting a spectacularly paintied gulf cart as a birthday present to Stern, suggested that it could help the violinist to "make the 100-yar trip from your home to your rehearsal studio, which you now make in your Buick," or that he could use it indoors to "save precious minutes spent running to the telephone."

"After all, Isaac," he said, "you're 60 years old now, and a little help should be welcome."

Mrs Carter returned to the presidential box , after a short speech that ended, "Happy Birthday, Isaac Stern." An hour or two later more than 100 formally clad guests were echoing her words, this time singing them to the familiar "Happy Birthday" melody. Stern stood speechless for a moment listening to the musical tribute and then retreated to a quiet table in the corner of the box tier at the Opera House, where his buffet meal was interrupted by a constant stream of well-wishers shaking his hand.

One of them, David Lloyd Kreeger, was stopped by Martin Feinstein before he could invite Stern to join him in an amatuer string quartet. "David," said Feinstein, "he's not going to play with you now. Forget it."

"He said no, but he didn't say never," answered Kreeger.

At the gift presentation in the Concert Hall, Stern had been so overwhelmed that he could say little more than: "This is a surprise. I am more overwhelmed than I can tell you." Later, at the party, he begun to come to terms with with situation: "I'm going to have so much fun driving that thing in the country. I'm going to leave it exactly as it is."

He was not quite so sure about the gift telephone. "With ITT footing the bills for tonight," he said, "I thought they might give me an international, remote telephone that I could use all the time without paying. But I think I'll have to put a quarter into this one every time I want to use it."

ITT president Rand V. Araskog, who was at the party, confirmed that his company and two subsidiaries, Hartford Insurance and Sheraton, had "helped to cover the expenses of some things that might not have been covered otherwise," but nobody suggested that his company might pick up the expenses for Stern's frequent and lengthy phone calls.

A few of these calls through nearly a quarter-century were remembered by Rostropovich. "Twenty-four years ago," he said, "my wife was giving birth to my daughter; I was in this country and Isaac was in Moscow. So he called me with news of my wife and my daughter, and I gave him news of his wife's health.

"I was in Paris when his friend Etienne Vatelot, the greatest living violin-maker, was celebrating his 50th birthday. So I called him and I said, 'Come to Paris, and we will give a benefit concert for Vatelot's birthday and give the money to help young violin-makers,' and he came. He came again when I wanted to give a concert for Chagall's 90th birthday, to raise money for the Chagall Foundation.

"And when he called me and said, 'Israel is celebrating its 30th anniversary, come and do a concert with me,' all I answered was, 'What time do you want me to be there?' Then, when I called him and said, 'I need your help for my orchestra's 50th anniversary gala concert, he simply said, 'Of course. What do you want me to do?' Nothing about how busy he was or how hard it would be for him to come to Washington."

"He is such a fine person -- such a fine musician," said Rostropovich. "When he speaks with his voice, it is the same as when he speaks with his violin, because his words and his music come from the same roots."