"I've made a little bit of history here," Ronald Reagan said yesterday afternoon on the South Lawn of the White House -- and then he blew it.

"I've nearly completed a public apperance without even once mentioning our tax proposal," he said.

"And it's not the place for it. I'm not going to tell you that we need a 25-percent reduction over three years, and we need it now. I'm not going to mention that."

The last part of the president's statement was nearly drowned out in laughter.

The event was historic, even if the president did not quite manage to avoid lobbying for his program. It was the American premiere of the recently discovered Symphony in F by the 9-year-old Wolfgang Amadeus Mozart, and it happened in a large yellow-and-white-striped tent on the South Lawn, with potted plants hanging from the text poles and a battery of 10 large electric fans on both sides of the tent battling the summer heat and the music. Besides the newly discovered symphony, the half-hour program included two pieces from Mozart's later years, the overture to "Cosi Fan Tutte" and the last movement of his "Linz" symphony.

Actually, Reagan was not the first person to mention economics during the proram. Leonard Slatkin, who conducted the program, introduced the Symphony in F with an oblique reference to the subject. "Not everybody on stage plays this piece," he said. "The reason had nothing to do with budget cutbacks. It's just that Mozart didn't compose this piece for flutes and horns."

He asked himself whether the symphony, which is probably the only Mozart composition now covered by copyright, was being played simply "as just a curiosity." His answer was that it is "an authentic, genuine mini-masterpiece . . . with one quality Mozart has most of, and that's humanity."

The 11-minute symphony will probably not find its way into Mozart's top 10, but it proved graceful, well made and extremely pleasant.

"Wasn't it lovely?" Ronald Reagan asked after the performance, and the applause was thunderous. The president thanked the Mostly Mozart program of Lincoln Center for bringing its festival to Washington, and quoted Mozart's friend and mentor, Franz Joseph Haydn, as saying that Mozart was "the greatest composer ever."

Besides paying tribute to the child Mozart, the president praised 17-year-old ballerina Amanda McKerrow of Rockville, who sat next to him with her parents during the concert. History was made, the president said, by her winning a gold medal in Moscow two weeks ago and "eight curtain calls from one of the world's most partisan and knowledgeable audiences."

It was a day for appreciating youth, he said, and "No one appreciates youth more than I do. I've had quite a while to appreciate it." At the reception afterward, the president stood in a receiving line in the East Room of the White House, which was considerably cooler than the tent outside, sipping orange juice from a glass held in his left hand while he shook hands with his right.

The guests, who included many leaders of cultural life from both New York and Washington, such as Avery Fisher, for whom the concert hall at Lincoln Center is named, and Marta Istomin, artistic director of the Kennedy Center, stood patiently in line awaiting their turn for quick handshake and presidential greeting. But Patrick Hayes, managing director of the Washington Performing Arts Society, drew Istomin out of the line and took her to the front, which he had reached, to intrduce her to the president.

"I'm just like Mayor La Guardia of New York chasing fire engines," said Hayes. "No matter how many times you've done it, there is still somthing special about standing two feet away from the president of the United States, regardless of politics."

Someone asked Martin Feinstein general director of the Washington Opera, whether he thought the young composer on the program had an future. "It depends," Feinstein said, "on what he writes after the age 9."

John W. Mazzola, president of Lincoln Center, said he was "fantastically grateful" for the invitation to play at the White House. "It's good for the city, for Lincoln Center and Mostly Mozart. The idea of taking Mostly Mozart to Washington is a good one, and I hope we'll be invited back again."

He said that "about half of the orchestra shook hands with the president, and all of them were just delighted to play at The White House. The president autographed the text of his speech and gave it to one of our violinists, Ruth Buffington. "She's really in the clouds."

Later, about 60 guests adjourned to the Kennedy Center for dinner in the Chinese Room before the concert, and impromptu toasts were popping like champagne corks. They were launched by Martin Segal, chairman of Lincoln Center, who thanked West German Ambassador Peter Hermes for helping to obtain the score of the new symphony for performance here and later in New York. Lincoln Center, he said, is "in the import-export business."

Rep. Margaret Heckler (R-Mass.) offered a toast "to the health of the arts," in response to Livingston Biddle's toast to members of Congress who have supported the arts.

Perhaps the best one-line joke of the evening was given by Daniel Boorstin, librarian of Congress, in response to a criticism of the Kennedy Center's architecture by Sen. Daniel Patrick Moynihan (D-N.Y.).

"Like democracy itself," said Boorstin, "the Kennedy Center is one of the greatest examples of the triumph of content over form."