There is "hidden talent" in the Washington bureaucracy, said Austin Kiplinger, a veteran watcher of the Washington bureaucracy who is also board chairman of the National Symphony Orchestra.

"I think," said Kiplinger at the party after the NSO's gala 50th anniversary concert last night, "we have so much hidden talent that it hasn't been revealed. In fact, it may be permanently hidden."

"Poppa Haydn!" Kiplinger interrupted himself, as he grabbed Mstislav Rostropovinch in a bear hug. He was referring to the evening's riotous encore, Haydn's "Toy" Symphony, which featured Rostropovich dressed as Haydn but fooling no one once they heard his Russian accent. It was the the talk of the party.

"Artistically I think they will be well-received, don't you?" said Kiplinger of the curtain-call orchestra.

Guests after the concert all trooped over to the Watergate Hotel for a massive party featuring a huge four-tiered birthday cake with the number 50 atop it. The party was the crowning event in a day filled with NSO anniversary observances, beginning with a White House reception and proceeding on to a cocktail party and dinner at the State Department. But after the concert, everyone, but everyone, was talking about Rostropovich and his encore stunt.

"I tell you, this guy is a showman," said Jack Gray, assistant vice president of At & t in Washington, to his friend.

"How did you like the maestro as a drummer?" asked Kiplinger, as he patted Leonard Bernstein (the drummer) on the back.

"We do our best for the cause," said Bernstein (who during the concert had accused Rostropovich of not giving him his cue to drum). "I had to learn the whole technique on stage."

"I couldn't keep up," said Livingston Biddle.

"You should have been a musician," concert-goer called out to Biddle as he left the Kennedy Center.

"Maybe I'll get a grant," said Biddle who happens to be chairman of the National Endowment For the Arts.

Flutist Jean-Pierre Rampal and violinist Issac Stern, who tried out new instruments during the Hayden selection, both thought that they would prefer to play the instruments for which they are best known. "I don't want to blow my own horn," said Issac Stern, who had soloed on ratchet, birdcall, and cuckoo at various points.

His bird call had sounded very dry and lacking in tonal richness until Rostropovich moistened it liberally from a gallon bottle of Smirnoff Vodka. Stern said that he had never tried applying vodka to his Stradivarius. "I know some violinists who have tried to improve their violins with vodka," he said, "but it doesn't help. Vodka slows down the vibrato terribly."

At least two living American composers -- Elie Siegmeister and Ezra Laderman -- were in the audience, but went unmentioned by Bernstein in the improvised speech he made while the Haydn number was being set up. Bernstein had said that no American composers attended the anniversary celebration because they were "too busy filling commisions and making money." Siegmeister and Laderman thought that perhaps their presence went unnoticed because they had bought and paid for their tickets rather than asking for free admission. Siegmeister bought tickets for himself and his wife as part of the celebration of their 50th wedding anniversary.

Also celebrating a birthday -- number unspecified -- was board chairman Kiplinger. While the symphony's enormous cake was being cut, a separate cake with one candle was brought out for Kiplinger and the crowd sang "Happy Birthday," not for the first time that evening.

At the White House earlier, the National Symphony got a cold buffet and a warm reception to its financial problems from Rosalynn Carter. The first lady said that the NSO "has earned a reputation for excellence, nationally and internationally." She gave credit for that excellence to "the leadership of such Washingtonians as Austin Kiplinger," the NSO board chairman who is spearheading a drive for funds -- particularly a $1-million grant proposal now threading its way through Congress.

"I hope the people of this city and around the country will express their support and admiration by giving generously to the symphony," said Mrs, Carter, and the audience applauded vigorously. Most of the 250 guests were members of the orchestra and their spouses, though the list included miscellaneous celebities such as FBI Director William Webster and composer-conductor Leonard Bernstein.

The refreshments at the reception had a musical motif. Some of the little cakes were cut and decorated to look like violins, others were decorated to look like violins, others were decorated with G clefs, and there were small, round orange cakes decorated with a treble staff and two eighth notes -- different notes on each cake. "Can you name that tune?" someone asked one of the musicians, pointing to a cake. "Let's see," he said. "The first two are C and A. The next two should be K and E. Where do you put a K on the staff?"

The room vibrated with the presence of NSO conductor Rostropovich, who was busily distributing bear hugs but restrained himself to a simple handshake for NSO conductor emeritus Howard Mitchell and a two-handed handshake when he greeted Mrs. Carter. "You must play my new work," Bernstein told Rostropovich. "I brought a score especially for you to see, but there's no time tonight and I must bring it back to New York tomorrow."

"Maybe tomorrow morning," Rostropovich offered.

With NSO percussionist Tony Ames, Bernstein offered a more detailed description: "It has eight movements, including a samba, a waltz, a turkey trot and a blues movement. It's not a percussion piece, but there's some great percussion in it."

A string quartet from the Marine Band sat in a corner of the East Room playing Mozart's "Eine Kleine Nachtmusik" (which normally requires at least five players) and across the room NSO president Martin Feinstein was trying to look unobtrusive while he conducted the music, out of sight of the players.

Some segments of the party developed into a tentative celebration of the proposed $1-million grant's progress in the Senate Finance Committee. Little knots of players gthered around Kiplinger, asking him about the appropriation's prospects and finding him cautiously optimistic.

"It looks like the committee will write it into a bill tomorrow morning if we bird-dog them all the way," he said. "But, as you know, there's many a slip."

Why, he was asked, didn't the orchestra try to develop more of an affiliation with the Smithsonian? "It was very complicated," he said. "We chose a simple and direct route."

"You know," he added (possibly unaware of reporters hovering nearby), "talking to Smithsonian Secretary S. Dillon Ripley is a little bit like talking to St. Peter. He seemed to have the feeling there were no performing arts activities at the Smithsonian."

Feinstein, asked what he thought of the prospects for the grant in Congress, said, "I belong to a non-prophet organization."

At a cocktail party and dinner later in the evening at the State Department, symphony supporters were enthusiastic about Rosalynn Carter's very supportive statements.

"I was surprised," said Henry Strong, who sits on the boards of both the NSO and the Kennedy Center. "The White House has an unwritten rule that it's not to be used for fund-raising."

Zbigniew Brzezinski also showed up in his tux at the State Department dinner, but failed to exhibit any ruffled feathers after his grilling by the Senate committee investigating Billy Carter. "I wasn't upset," he said about his experience before the committee on Wednesday. "I merely made a point that had to be made. But I'd be delighted to go back because I thoroughly enjoyed their company."

Greeting the guests were Chief Justice Warren Burger and his wife along with Austin Kiplinger and his wife. "I played the bugle at Boy Scouts and then graduated to the cornet," said Burger when asked about his musical interests.

"You played swimming call?" asked Kiplinger.

"Oh, yes," said Burger.

"You played mess hall?" asked Kiplinger.

"Oh yes," said Burger.

Board member Strong was realistic about both the orchestra's financial progress and the musicians' role in it. "If it comes to crunch, it will be difficult for them professionally," he said of the situation. "Something's got to give. I don't think it will be salaries of musicians. But we may have to go to them and say, 'How you can help us?

David Kreeger, who sits on many boards, was out on the terrace enjoying the early evening cocktail party. "What's new? Well, the opera's doing beautifull," said Kreeger, who is the opera's president. "Subscriptions are up -- they're 85 percent sold out." t