This week Mstislav Rostropovich has been conducting the Premiere performances of the newest work by noted French composer Henri Dutilleux, an unusual orchestral score called "Tones, Space, Movement."

It is always interesting to know if a composer is satisfied with the way a new work sounds when he finally hears it for the first time.

On this occasion, Dutilleux said he was very happy with the National Symphony and Rostropovich's part in the work. "Rostropovich is amazing in rehearsal," he said. "Superb, and the orchestra is excellent."

"There are some slight changes I want to make," he added. "But I am very happy."

The premiere has brought together two musicians who have greatly admired each other since they first met.

Dutilleux recalled that meeting in an interview. "It was around 1960 when one day Igor Markevitch (composer and conductor) said to me, 'You must come to a concert tonight. I will not tell you more about it. You must be there.'

"The concert," Dutilleux remembered with a smile, "was a cello recital by Rostropovich. Not long after that Markevitch suggested that I should writer a piece for cello and orchestra.

"But it was not until later when Rostropovich urged me, that I wrote for him the Cello Concerto, which he played for the first time at the festival in Aix-en-Provence in 1970. After that he was going to play it in this country - with the Boston Symphony - but then he was having problems in Russia and could not leave for the United States."

The composer, whose works have been played in this country by the late George Szell and Charles Munch, as well as by Eugene Ormandy and Howard Mitchell, continued: "Then Rostropovich asked me to write something for the National Symphony and I was delighted to do so. (Antal) Dorati had wanted me to write for the orchestra for the orchestra for the Bicentennial, but I was busy and did not have the time.

"Then while I was working on this piece - the Timbres , escape, mouvement - I had family problems, that occupied my time, and I did not finish it. But it will be finished and Rostropovich will play the entire work next season."

In the meantime, Dutilleux has written a string quartet on commission from the Koussevitzky Foundation in the Library of Congress. The Juilliard String Quartet will give the premiere of that work, which is called "And Thus the Night," in April, an event that will bring Dutilleux back to Washington.

Dutilleux's music continues to make its way in a fairly steady stream on American recordings. Not a prolific composer by any means, Dutilleux, who will be 62 next week, has always written rather slowly and with great attention to detail.

At a time when the name of only one other French composer, Olivier Messiaen, is at all known outside of France, the music of Dutilleux, once vigorously championed in this country by Szel and Munch, seems likely, thanks again to Rostropovich, to be moving again to the foreground.