Mstislav Rostropovich and Leonard Bernstein were full of excitement at the final rehearsal for last night's concert, for which they had been preparing the orchestra over the previous five days.

Bernstein told his listeners at a press conference afterwards, "The orchestra played an hour overtime on Friday, and two hours on Monday, after a double rehearsal. That means from 10 a.m. until 5:30 in the afternoon. And not one person got up and left. The musicians have the right to leave when you call for overtime. It is one of the marks of a great orchestra when the players want to be great.No one left."

He then repeated what he had said earlier to the orchestra. "There is a story in Newsweek this week, about your orchestra and Rostropovich. It is very complimentary. But someone in the orchestra, who asked to remain anonymous, is quoted as saying that the orchestra and conductor are on trial, that they have to prove themselves, that this is, after all, still a second-class orchestra. That," said Bernstein with emphasis, "is not true. This is a first-class orchestra, and don't ever forget it."

he famous composer-conductor had just finished putting the final touches on Three Meditations from "Mass," music he wrote six years ago for the dedication of the Kennedy Center, now to be heard for the first time with solo cello, and orchestra; and on "Songfest," his new cycle of 13 songs to American poems, for six singers and orchestra.

He has heard Rostropovich not only as the solo cellist in the Meditations, but also as the conductor of the premier of the new Bernstein overtune, written and named "Slava" in honor of the new NSO conductor.

As Bernstein led the Meditations, he kept turning to his famous host-so-loist and nodding approvingly. When he finished them, looking pleased, an expression of surprise crossed his face when Rostropovich said he would like to repeat them.

"What?" You want to do them again?" he asked. "But we won't have time". Then, with a gesture of resignation, mixed with a certain pleasure at the prospect of added polishing, he started in with the tricky second one again.

"I knew this would happen," he murmured in the admiring tone of one genius facing another. And where the music had been lovely before, it was now breath-taking.

"What has happened here in - how long did you work with the orchestra before I came?" he asked the National Symphony's proud new music director." One week and then a week with a concert?" It's a miracle. This is a first-class orchestra. The last time I was here it was still perhaps a second-class orchestra playing better than that"

At one stopping point while Rostropovich was rehearsing the new overtune, Berstein came up to the stage and said to the orchestra, "Take out "Pooks' You don't say it very well." Threatened with the removal of its big moment - a shout of "Pooks!" - the musicians protested, "Leave it in. We'll shout it!"

And sure enough. Where the first time around, they had let out a rather feeble sound, now they really shouted. So "Pooks" stays in, along with a tape soundtrack gongs, ratchets, whistles, and everything else 10 men on percussion can handle.

"Slava" was written very recently, and in a few days, though some of its material was heard a couple of seasons ago in the ill-fated "1600 Pennsylvania Avenue." Rostropovich, who admitted at the press conference that he had never before conducted anything in the jazzy swinging style of the new overtune, ripped into it as if he had been born and raised on Manhattan's West side.

"I love it," he said. "I love very much the music of Maestro Bernstein. And I am so proud today of my orchestra - to play three world premieres of Bernstein in one concert. That makes an orchestra play better."