ONE EVENING late in December in the year 1971, I stood outside a door in the Watergate Hotel listening to Mstislav Rostropovich, on the other side, playing three notes over and over on his cello. They were not difficult, but he was working at them very hard. I had gone to his room to pick up a letter that he had asked me to carry from him to Pablo Casals on Casals' 95th birthday, which was soon to be celebrated in Puerto Rico.

"Those three notes you were playing over and over," I could not resist asking. "Why were you working like that when they sounded very easy?" With a look of complete honestly, Rostropovich answered, "But I have to practice!"

Rostropovich is now, of course. the music director of the National Symphony, a post no one foresaw for him back in 1971, But he is still a cellist, and in a recent conversation was delighted to talk specifically about the beauty and challenge of his instrument, and the goal of "more perfection" toward which he plans to work for the rest of his life.

Does anything remain difficult for him? His answer is quick and easy: "No, not difficult. I do not have real technical troubles. But of course each passage I must make more perfection as I can." The memory of those three notes sounded clearly in my head, and I remembered one sentence from the letter that went from Rostropovich to Casals: "You are the Olympus toward which we all strive."

How old was the virtuoso when he began? "I was 8. I changed cellos three times. First was very small cello; after that was half cello; and also I paly three-quarter cello. Normally it is very dangerous for small boy with small fingers to play cello. For me, when I was 8, big cello would be like double bass."

By now, Rostropovich has played nearly everything written for the instrument, either solo or with orchestra. He has also been the inspiration for some of the best compositions of Shostakovich, Prokofiev, Britten and other contemporary composers. But there are some things he has played either never or very infrequently. "I have played only works in which I have interest. I must tell you that I have played very difficult works two or three times, then not more. For instance the Glazunov Concerto - it is very difficult - I have played it twice, then not again. It is not a real great composition, not like his violin concerto."

There are things that seem to set the Rostropovich playing apart from all others today: the size of his tone, his phenomenal pianissimo sounds, his total physical control of the instrument. He has reasons why these come out as they do:

"For my sound, I want to make my hand as near to my body as possible. If you are too far, your strength is lost, that's a physical rule." (At this point, having taken out an old cello formerly used by his daughter Olga, who now plays a former cello of her father's, Rostropovich began to illustrate.)

As the incredible tone poured out of what he called "a lousy cello," he pointed out, "That's coming from here, from shoulder. That's pretty heavy," emphasizing the weight he was putting on the bow. "If I take a cello from another cellist and want immediately to play a little bit, always I must make twice more tight the bow."

"I like very tight bow. Why? Because I make real big pressure." Is the bow always tightened the same amount, for all music? "Yes, yes, always, always the same. You know, of popose," he explained further. "It is possible to produce not only a very different dynamic, but also a different sense of sound.

For example," and again be demon-icated, "I make only two notes, for music of Each." Out came a golden line of melody that ran through four or five instead of just two notes. The texture, character of the tone, the rate of the [WORD ILLEGIBLE] were flawlessly shaped to create the spirit of a Bach sarahande.

Suddenly the sound changed. "But now for Schubert or Schamann." Instantly the virato was slightly faster, the tone more intense, though on the [WORD ILLIEGIBLE] notes as before.

"That's much more warm," was the way he described the second phrase. "But not Bach. Why? Because Bach more objective, much schelduled, much tough, much bigger. That's why, always, for each composer I try to produce a special sound."

What about vibrato, that subtle, vital ingredient the acquisition of which is such a problem for student cellists?

The great cellist's face took on the look of an apostle as he burst out enthusiastically, "For me, vibrato, that is the real color. Normally for low string. I make the vibrato a little bit slow. That's natural for low string physically, low string have some natural vibration. If I vibrate on A string. higher string. highter register, that's much faster vibrato, slower, coloratura for example, much faster vibrato.

"You must make the vibrato with any finger, little finger. also with the thumb. Sometimes people do not understand how expressive is vibrato. I do not like vibrato in woodwinds. If the flute makes a slow vibrato that's like - " Here the cellist had to use his voice to imitate a kind of seasick up-and-down humming. "In high woodwinds, I like something like cold beauty. But in strings, warm beauty!"

"For Bach," he went on with one of his favorite subjects, "it is impossible make some vibrato like in Schumann. That's because of the music. It's absolutely different." He concluded with emphasis, "Vibrato is most important for expression."

To Rostropovich tone color of which vibrato is a part, is of primary importance. "Normally string tone comes from the finger. here." He held up his left hand and pointed to the exact spot where, most of the time, the fingers rest on the strings.

"But for different kind of sound very possible here, and here, and here," he said, as his hand moved across the strings. "I show you. in Debussy Sonata." He played a three-note sequence of G.F. sharp, F, and back up the chromatic phrase again. He had produced a normal, bright tone. "But" he paused for a moment, and placed the flat part of his finger on the string. "I want a sound like an old flute." Out came a sound that could be called noted, rather briefly and velled. "Also the bow can be played here on the string. and here, and here. Like a painter, the artist has many different colors."

In a recent master class at Catholic University, Rostropovish had advised a student to use the whole bow, not just a third or so fo it. "What about the whole bow," I asked.

"The whole bow is not for every-thing," he answered. "That depends on the music. And for the hall where cellist is playing. For example, if now good cellists are playing for chamber audience, perhaps 100-200 people - you know I started here in Washington in Lisner Auditorium 25 years ago, perhaps 200 people - he does not always need whole bow.

"But now you play for 2,000 audience, there must be a little bit more. That's the first problem for the cellist when you come into a big hall: firstyou think about last row. You must make service for these people. I have the feeling that music is not coming until it gets to this last row. That's why I always control my piano (soft) playing. I like piano playing very much. I adore piano playing. That's my beloved nuances. But, in each hall I control so that it comes to the last row. In each different place I play differently.

"You know about halls," the well-traveled performer continued. "Sometimes ia hall makes the reverberation; it makes your sound more beautiful. And sometimes if the hall acoustics are very dry then your sound comes - " There was one of his short pauses as he sought, and found, the right words, "without dress. And that's sometimes good for per-former. You can compensate for that a little. You must make your control very carefully. In a good hall you do not hea if some small thing is not perfect. In a bad hall, you hear it like a knife."

When it was announced that Rostropovich would become music director of the National Symphony. some peole. who claim him as the world's greatest cellist, were afraid that he would give up his own playing.

"No, no, no," he cries at the idea. "I will always play the cello. That's, you know, cello is my soul. When I am not conducting. I play the cello absolutely twice as much. That's my contact with a big repertoire, big symphonies, my direct contact with great music. That makes for my imagination so many new ideas for sounds for the cello. Also my cello playing I need for my conducting."

"I like the cello as near my body as possible. Look. it rests right there." As he put the instrument in its accustomed position, he added, "I embrace cello like a beautiful woman. That's very easy if you love cello." About the second instrument in his life. the piano, which he plays like a master, while insisting what he never has time to practice on it, Rostropovich says, "I love piano! It is my other instrument. But the cello - that is my soul!"