hen he was not playing the piano tonight in Carnegie Hall, Dimitris Sgouros looked liked a shy, nervous 12-year-old boy. He clasped his hands together during intervals when the piano was silent--relatively rare intervals in Rachmaninoff's Third Piano Concerto--and swayed his body gently from side to side looking very tiny on the piano stool with a giant Steinway in front of him and the National Symphony Orchestra behind him.

He kept a handkerchief tucked away inside the piano and took it out frequently to wipe perspiration from his face. It may be that the temperature in the hall had been raised by the heat of his spectacular virtuoso performance.

When he took a bow, as he had to do repeatedly for nearly 15 minutes tonight, with interruptions for two encores, he seemed to shrink into himself, almost afraid of the intense reaction he had provoked in such a large crowd.

A near-capacity audience gave him one standing ovation after another, and its applause, which was already loud, rose to a roar when Mstislav Rostropovich engulfed his small piano soloist in an enormous bear hug. The reaction was well earned; Sgouros has a natural musical talent of still incalculable dimensions.

The printed program had promised the audience an all-Tchaikovsky evening, but two Tchaikovsky items were dropped the day before and his "Manfred" Symphony moved to the beginning, to make room for Sgouros and Rachmaninoff. It was the first time the pianist had played the concerto with an orchestra, but it sounded as if it might have been the hundredth.

The diffident child disappeared and a totally different person took over whenever Sgouros' hands came near the keyboard. He was self-assured, dominant, able to whip an entire orchestra into submission with the speed and strength and accuracy of his hands, the passion of his interpretation, and sometimes, though not always, the sheer musicianship at his command.

He is not yet a complete musician--he has still things to learn, particularly about pianissimo playing--but even in those aspects of the art that usually come in later years, subtlety in the dynamic gradations of a phrase and little but meaningful variations of tempo, he already shows occasional flashes of insight far beyond his years.

The Rachmaninoff Concerto does not tell a listener everything that needs to be known about a pianist, but it is a very thorough test of technique and facility in the romantic style of performance. I still wonder what he might be able to do with Mozart, but I heard enough tonight to say that a major talent made its American debut with the National Symphony in Carnegie Hall. If he does not burn himself out in his teens (as could easily happen if his career and further development of his talent are not carefully managed), he should be one of the dominant performers of the early 21st century.

That was what Zubin Mehta had in mind when he rushed backstage after the performance asking, "Where is that boy's mother?" His intentions might have been misinterpreted, since Sgouros' mother is also his manager and agent. Mehta was not looking for a new star to play with his orchestra, at least not yet. When he found her, Mehta said, "Make him study--no more concerts for a while."

The sentiment was echoed by Henry Fogel, executive director of the National Symphony. "Of course he's not a completely developed artist yet. What this debut should do is help him to find a patron who will support his education. The family doesn't have much money, and the temptation to cash in now can become very strong."

That may have been what Rostropovich had in mind, too, when he brought one of the distinguished guests who flocked backstage after the concert over to meet Sgouros, who was standing in a corner shielded from the stream of admirers by a small table. "This is Alice Tully," Rostropovich told the young pianist. "She has done more to help young artists than anyone else in the world."

Also in the audience were Rohan Joseph, music director of the American Philharmonic, and violin virtuoso Erik Friedman, who gave concerts with Sgouros last summer during a musical cruise aboard the ocean liner Azur. During intermission, before the young Greek came on with the Concerto, Friedman warned a friend who was sitting in a box tier, "Don't sit too near the edge. You are about to be knocked out of your seat."

In Tchaikovsky's "Manfred" Symphony, which opened the program, the NSO achieved a quality of sound that I have never heard it make in the Kennedy Center. The bass strings and winds were extraordinarily rich, the brass incisive, the balances beautifully clear, and the smallest nuances delicately articulated.