Shortly before cellist Nathaniel Rosen became the third American to win a gold medal at the International Tchaikovsky Competition last July, he took advantage of some expert advice: He and Mstislav Rostropovich met in an English village, in the library of the late Benjamin Britten.

Rosen, who was then first-chair cellist of the Pittsburgh Symphony, had set his mind on winning the 1978 contest, after losing earlier.

"The meeting was actually on my way to Moscow," Rosen said yesterday before his performance at the University of Maryland.

"I'd called Rostropovich on impulse in Washington from Pittsburgh. I told him I was entering and asked for his help. After all, he is not only the greatest living cellist, but he was also a judge in Moscow time after time.

"First, he invited me to Washington, where he gave me some advice - on things like range, on the need to more explicitly characterize the music and on the necessity to be rhythmically more compromising.

"Then, when he heard that the Pittsburgh orchestra was going to England, he said to come to Aldeburgh for more work.

"I asked how to reach him there," - in the small village where Lord Britten had lived and created one of the world's leading music festivals - "and he said 'In Aldeburgh, all you need to do is walk up and down the street and scream 'Slava' until I hear you.'"

Fortunately such a ruckus was forestalled.

"He was conducting that night," Rosen recalled. "And then there was a party afterward. So we worked from 12:30 a.m. to 2:30 a.m. in the Britten home, which is still occupied by Sir Peter Pears."

Rosen's subsequent Moscow triumph lacked the media hype that greeted that most famous of winners, Van Cliburn, in 1958 - no ticker-tape parades, though there was a White House reception. It came at an advantegeous stage in his career: He was 30, he had a full repertory, was well established in his profession and didn't have to worry about some of the problems of premature exposure that have dogged the otherwise sensational success of Cliburn, for instance.

"Of course I wouldn't have turned it down earlier," Rosen said. "But now I am at ease with myself and have a feeling I can lick the problems. I'd always planned a solo career, but working on it at this point makes me comfortable with the deluge of offers that have come."

He performs three separate programs on Washington stages alone this year. Yet he's not trying to do everything at once. "For instance, I've taken on the first three Beethoven sonatas, but not the last two yet."

Is he attracted to staying with orchestras, at least part-time?" "It's not for me," he utters unhesitatingly. The Moscow win allowed him to live in New York as a soloist, and he likes life there, with his wife, Jennie, also a cellist.

Rosen wants to become a musician's musician. The ideology of Casals or Rostropovich is not something he plans to be involved in.

"My life is really very simple. My attitude toward my career is that I want many people to ask me to play, and that is the luxury that I want the most."

In Moscow, Rosen was one of 280 competitors from 37 countries.

"It's not really a live-or-die situation," he observed, "but winning has made things much easier. I would have left the Pittsburgh Symphony anyway, despite my fondness for (conductor) Andre Previn. But with such a stroke of good fortune, I have been able in this season to develop my career at a pace that might otherwise have taken many years."

Rosen in Concert

One year ago Nathaniel Rosen became the first American instrumentalist since Van Cliburn in 1958 to win a gold medal in the prestigious Tchaikovsky Competitions in Moscow. Last night in an unmercifully sweltering Tawes Theater at the University of Maryland, Rosen gave an enthusiastic audience the full measure of the gifts that won him that prize.

Neither a string breaking early in the D Major Solo Suite by Bach nor perspiration dripping steadily onto his cello deterred Rosen in displaying every kind of brilliance in technique and interpretation.

In the most elusive of the Bach suites, the most difficult to sustain, Rosen offered beauty of musical thought in pacing the courante, while he made the gavottes dance. Earlier in a sonata of Francoeur and the A Major Sonata by Beethoven he had given elegant proofs of the noble sound he can draw from his instrument. He also demonstrated a wonderful right arm in gradations of tone that breathed life into the music.

For his pianist Rosen was unusually fortunate in Jonathan Feldman - the more he played the more he was to be admired for the superb control of all that he touched.

Schumann's Adagio and Allegro had pensive poetry and great dash in the proper degrees, and the closing romp by Bohuslav Martinu on a theme from Rossini's "Tancredi" was a ball. That both artists did what they set out to do dressed in white tie, tails and vest was astonishing. Their later appearance minus the coats was a step in the right direction. Audiences - like performers - are happier, as Rosen said to his hearers, when they can be more comfortable. CAPTION: Picture, Nathaniel Rosen in practice, by James A. Parcell - The Washington Post