It took 50 years, but the National Symphony finally fielded a "world-class" orchestra last night. It happened just when everyone thought that the orchestra's gala 50th anniversary concert was over. After all, Isaac Stern and Mstislav Rostropovich had finished a bang-up job on the first movement of the Brahms Double Concerto -- what more could anyone ask?

Suddenly -- was 11:20 -- Leonard Bernstein appeared on stage. "Don't leave," he ordered the auddience. "The evening is young. We have a surprise. It may go on until one o'clock." At that point Bernstein could no longer stand the script he had been handed. "This is not my material," he explained. "Slava gave it to me." Seeing the stage hands rushing around to clear the stage he begged them. "Don't rush. We have to stall."

Then, returning to his script, he announced that of all the composers the National Symphony had invited to its party, only Haydn had accepted. "St. Peter has given him permission to come down for a while, and here he is!"

On walked a white-wigged, velvet-pantalooned Haydn, in silk stockings and patent leather pumps. In a voice with a strong Russian accent, which turn out to be exactly like that of Rostropovich, "Haydn" began explaining that, while he had seven musicans (out came seven of the NSO players, all with wigs) he needed some volunteers. As he began calling their names, he took an instrument out of a large yellow bag: Leonard Bernstein, best instrument for him -- drums! Is so difficult! Zbigniew Brzezinski -- for you most important instrument, kazoo. Senator Charles Percy" [R-Ill., a fact "Haydn" did not announce] "enormous loud cymbal. Leonard Bernstein is for musicality. Zbigniew Brzezinski for security.

"Rampal come here!" Explaining to the audience that "Rampal always say flute not loud enough," the celestial visitor handed him a ratchet, which Rampal immediately began twirling in loud delight. Isaac Stern, here, come! rWhistle." At this point Haydn picked up a magnum of vodka from which he gave moral support and encouragement to his players throughout the ensuing music, sometimes spilling large quantities of it on Stern.

"Liv Biddle," and for the director of the National Endowment for the Arts there was a cuckoo, which the director seemed delighted to play. For S. Dillon Ripley, secetary of the Smithsonian Institution, a triangle, and Carter Brown was ordered to the stage to hold it for Ripley. And finally for Ambassador Hermes a bird whistle.

"First we have short rehearsal," Haydn explained. "Performance will follow soon." Well friends, let me tell you: Zbigniew Brzezinski's talents are being wasted these days in whatever job he is holding down. As the operator of the kazoo, he turned in a performance whose virtuosity matched anything heard earlier in the evening.

And that is saying a lot when you realize that the program brought out Galina Vishnevskaya in dazzling form as she sang Tatian's Letter Scene from "Evgeny Onegin." And Rampal playing the flute while conducting a chamber ensemble of the NSO in a Mozart to please heaven itself. And a trio of friendly musicians named Stern, Rampal and Rostropovich sitting down to enjoy a Haydn Flute Trio.

The concert began with a rip-roaring performance of "Slave!" the overture that Leonard Bernstein wrote for Rostropovich's first season as music director of the National Symphony. It was a timely diversion to hear its taped voices hollering, "I give you the next president of the United States!"

Rostropovich, getting the utmost brilliance from the birthday players, added a blazing Roman Carnival Overture later on, and tremendous dramatic power to the Letter Scene. Switching to the piano, he joined with Bernstein in Mozart's C Major Piano Sonata for four hands, in which again there was the feeling of two intimate friends enjoying making great music together. He also supporters. Melvin Earl-Brown has a countertenor of tremendous power and remarkable rang. It would be difficult to say, on the evidence of Ratmir's aria from Glinka's "Russlan and Ludmila," just what his future in the clouded countertenor world may be.

Hugh Wolff, conducting the Brahms concerto movement, add new strengths to those that have already marked him as a man of great gifts. The sound and playing of Stern and Rosstropovich in the Brahms was filled with fire and often luminous beauty, quantities Stern's violin also gave to the Haydn. All this was but prelude to the coming of the unique, not to say unprecedented, if not indeed world-shattering orchestra composed of Messrs. Bernstein, Brzezinski Percy, Rampal, Stern, Biddle, Ripley, Brown, and Hermes. With their accession to the ranks of the National Symphony, Washington's orchestra now has no rival anywhere in the world. Happy Birthday NSO, especially to its newest players!