NOBODY DENIES that Mstislav Rostropovich is a great musician. But he raises a question every time he raises his baton, and it will be asked again Tuesday night, when he returns to the Kennedy Center and begins his sixth season as music director of the National Symphony Orchestra: After five years with the NSO, what kind of a conductor is he?

In the last five years, the genial Russian emigre has become a familiar and beloved part of the Washington landscape. His special brand of fractured English, imperfect but eloquent, often curiously poetic and oddly precise, has inspired countless anecdotes and imitations. So have his legendary appetites for music, food and vodka (Stolichnaya, kept in a freezer and served to his amazed guests in brimming water tumblers). He has distributed bear hugs lavishly and spectacularly to hundreds of Washingtonians at pulic gatherings. More serioisly, he is remembered for his courageous defense of Aleksandr Solzhenitsyn, the Nobel Prize-winning novelist whom he took into his home and sheltered after the Soviet government had made him a nonperson.

Because Rostropovich is in Washington, this city has been the scene of special events -- galas and birthday celebrations -- involving many of the world's greatest musical personalities: Isaac Stern, Nicolai Gedda and Aaron Copland, Jean-Pierre Rampal and Leonard Bernstein -- the peers of Rostropovich. Rostropovich's performances of the "1812" Overture, outdoors with a battery of real cannon, are spectacular events that attract almost as much attention as major music festivals and competitions. He has become, in a way, a national monument: our first national monument that speaks with a Russian accent.

Rostropovich the conductor is very much like Rostropovich the man: mercurial, energetic, full of enthusiasms that he sometimes communicates with an amazing intensity. Like his cello (which he will be using in this week's NSO program), his baton expresses his personality -- though perhaps not quite so subtly, not with the same seeming ease and spontaneity, not in the same lavish and colorful detail. Not yet.

As a cellist, there are millions of fans and hundreds of critics who call him the world's greatest. From Paris to Tokyo -- anywhere in the world except the Warsaw Pact countries, where he cannot go (the Soviet Union deprived him of citizenship in 1978) -- Rostropovich and his cello draw standing-room-only audiences. As a conductor, his performance and the audience's reaction are less predictable. He can be one of the world's best conductors or he can be mediocre -- sometimes on the same evening. That happened at the end of last season, when he conducted an unforgettable performance of Prokofiev's "Alexander Nevsky," preceded by an undistinguished Brahms Fourth Symphony. In bygone seasons, he has produced readings of such composers as Beethoven and Schumann that ranged from eccentric to just plain bad.

Still, his good evenings can be spectacular. His performances of the Britten "War Requiem," music from Prokofiev's "Romeo and Juliet," Tchaikovsky's "Iolanta" and Shostakovich's Fifth Symphony (to name only a few memorable occasions) were once-in-a-lifetime experiences. The familiar NSO seemed to be replaced by a new and much greater orchestra, functioning at a higher-than-usual energy level, willing to take chances and succeeding brilliantly, playing with unprecedented clarity, richness and precision -- above all, with total involvement in the music.

Only one other conductor regularly associated with the NSO is able consistently to elicit this kind of playing: Rafael Fru hbeck de Burgos, whose special chemistry with the orchestra has made him its principal guest conductor. Otherwise, Rostropovich's best work with the NSO can be compared only to the giants who come occasionally as guest conductors: Abbado, Bernstein, Tennstedt.

To name him in such company is to indicate a tremendous potential. On occasion (and the occasions multiply with each passing year), that potential becomes a living reality. At this point in his career, Rostropovich is a specially exciting conductor because the potential is unquestionably there at all times, and you can seldom be sure when it will slip into high gear. It is fairly safe to predict that Shostakovich's 14th Symphony will be an exceptional event on March 8, 1983. But what about the "London" Symphony of Vaughan Williams on Feb. 1? Or Schubert's Ninth on March 1?

Last season, it could have been predicted that his interpretation of Tchaikovsky's "Manfred" would be striking and individualistic -- Rostropovich almost always is, and never more than when the composer is Russian. But it could not have been predicted that he would communicate that interpretation so effectively to and through the orchestra. And nobody could have forecast the pure magic he found in Mendelssohn's music for "A Midsummer Night's Dream."

WHY THE unevenness? Rostropovich has been busy learning to handle a new instrument and its vast, complex repertoire: the symphony orchestra. His new instrument is 106 human beings, not four strings, and all of a sudden he is expected to be an expert on everything from "Eine Kleine Nachtmusik" to "Also Sprach Zarathustra" -- mostly material that has never involved him before.

Whatever his eminence as a cellist, at 55, Rostropovich the conductor must still be considered a promising talent -- often brilliant, always unpredictable. He had been an occasional guest conductor in a limited repertoire for a long time, and that is a challenge, but it is in no way comparable to the awesome responsibilities of a music director. A free-lance conductor with lots of charisma can make a good living with a portfolio of perhaps 3 dozen works. A music director has to know it all, and that is only the beginning of his job. He must be a charismatic leader, a teacher, a publicity expert, and in the United States, he usually has to help with fund-raising. Unlike the glamorous guest conductor, he has to live with an orchestra year after year, handling routine chores as well as crises, auditioning new players and boosting the morale of old ones, representing the orchestra at social functions and negotiating with its board of directors, persuading big-name soloists and conductors to appear with his orchestra and helping it to get a fair share of the action on radio, television and records.

A guest conductor is like a hotel patron whose main concern is the quality of the service. A music director is like a householder who must worry about plumbing and wiring, the age of the wallpaper and the stability of the foundations -- not to mention next week's menus and next month's heating bills. It is the difference between marriage and a one-night stand.

As a cellist, Rostropovich was a perennial tourist; a denizen of hotels and intercontinental jets. The offer of a music directorship must have come unexpectedly, and it implied a drastic change of life style -- probably more drastic than he originally imagined. After five years at the NSO, his fixed address is still an apartment in Paris -- more conveniently located, on the whole, for an international free-lance musician. When he is in Washington, he still lives at the Watergate. But he has bought a home in this country -- near his favorite Russian Orthodox monastery in upstate New York. He still hopes to be able to return to Russia, but his roots are sinking deeper in the United States, specifically in Washington. It may be relevant that he choose the stability and drudgery of a music directorship when it was becoming apparent he would no longer have a homeland. In a sense, the National Symphony has become his new home, and he manages it with all the care and devotion of a suburban mortgage owner trimming his lawn, painting his gutter and worrying about termites.

There have been similar changes in his conducting style since he made this basic, long-term commitment. In a sense, he seems to be mellowing. In the first few years, his interpretations sometimes tended toward extremes of tempo and dynamics, or (for no perceptible reason) he would emphasize one of the inner voices usually buried in an orchestral texture. He would push the orchestra up to and beyond its limits. More recent seasons have brought no lessening of intensity but a more careful use of it. There is a growing sense of subtle dynamic nuances between fortissimo and pianissimo, fine details of orchestral balance and the things that music can say at moderate tempos. What this means, really, is that Rostropovich has been exploring the expressive possibilities of his new instrument.

At the same time, he has been exploring its repertoire. Frankly, it would have been easier if he had been born in Germany or Austria, where most of the music that is still the bread and butter of American orchestras was composed. Russian performers traditionally have trouble with classical German and Viennese repertoire, particularly before middle-period Beethoven. They are born and educated in a tradition whose roots go back only to the mid-19th century, and Bach, Mozart and Haydn seem to be almost a foreign language to them. David Oistrakh's Bach, for example, was beautiful, but not really Bach. Some Russian musicians are exceptions to this rule -- chiefly those who emigrate or spend a considerable time outside the Soviet Union, such as conductor Rudolf Barshai, pianist Vladimir Ashkenazy and perhaps, eventually, Rostropovich.

Rostropovich embodies the Russian tradition as thoroughly as any musician alive, including its limitations. But as a cellist, he has ventured successfully into 18th-century repertoire -- notably Haydn and Boccherini; Mozart wrote nothing significant for solo cello. One of the reasons he is planning to take a sabbatical 1984 is reportedly to come to terms with the Bach Suites for unaccompanied cello. This will be, predictably, an epic encounter, as was his recording of the Beethoven cello sonatas nearly 20 years ago in partnership with Sviatoslav Richter.

But orchestrally, the 18th century is still relatively unexplored territory for Rostropovich. His mastery of the orchestral Beethoven has come slowly, but those who have been watching this small corner of his repertoire steadily for the last five years have seen a fascinating, even inspiring, process. His Beethoven, at least in the Fifth, Seventh, Eighth and Ninth symphonies, has improved tremendously in recent years. It is not at the level of the specialists who grew up in that tradition, and perhaps it never will be, but it is in the mainstream now; unlike his English, Rostropovich can now speak Beethoven without a perceptible Russian accent.

Mozart, Beethoven and Brahms are crucial, of course, but there are other values in music. If one kind of music specially inspires Rostropovich's genius as a conductor, it is the music of Russia; he describes himself, in his exile, as "an ambassador of the Russian people -- not their rotten government -- and Russian music." His particular strength lies in music of the Russian generation before his own: primarily Prokofiev and Shostakovich. It is peripheral to the central concerns of American orchestras, but it is a great repertoire, an exciting, colorful repertoire, and Rostropovich is systematically working to impress its greatness on the world. It embodies the forlorn dream of what could have remained a great creative culture in a more benign political climate. If all of Rostropovich's ambitions are achieved in the next decade, the world's greatest Russian orchestra will be the one headquartered in Washington, D.C. Translating that paradox into reality is worth considerable effort, even if it can be only a part of the assignment for an American music director.

Rostropovich's strength in Shostakovich and Prokofiev may actually be part of a larger phenomenon: he is always specially eloquent and effective in the music of composers he has known personally. Fortunately, because of his long-standing international stature as a performer, that list includes noteworthy living composers from Bernstein to Lutoslawski and such great figures of the recent past as Benjamin Britten.

It may be--as the available evidence seems to indicate -- that Rostropovich has a special affinity not only to the music of friends but to modern music in general; more information on this point will be forthcoming in the next two weeks as he conducts the world premieres of several works by Scandinavian composers. This knack for modern music is shared by at least two other conductors who were born outside the geographical mainstream of Western classical music, Seiji Ozawa and Zubin Mehta, and it may be a clue to a special role played by early environment in forming the tastes and talents of a conductor.

In older repertoire, there may be an advantage in early immersion in a tradition -- the absorption of certain kinds of harmony, rhythm and melodic cadences with your mother's milk, long before you begin to think consciously about such things. In modern repertoire, all conductors approach the music on an equal footing; it is almost a laboratory test of pure musical intelligence and instincts. On these points, most members of the orchestra, who are the severest critics of conductors, give Rostropovich high marks. "He learns very quickly," says one NSO player. Another puts the same basic idea less positively: "Sometimes he seems to be learning a piece at the first rehearsal -- but after that, he knows it."

CAN A GREAT cellist become a great conductor? The obvious answer to this question is a single word: Toscanini. As far as the National Symphony is concerned, Rostropovich is a part of a tradition that began when cellist Hans Kindler founded the orchestra more than half a century ago. All of the orchestra's four music directors, except Antal Dorati, were cellists before they became conductors. The conductor Rostropovich mentions most frequently as his ideal, Serge Koussevitzky, played the closest possible thing to a cello: a double bass. He also spent a quarter-century with the Boston Symphony Orchestra and made it one of the world's greatest. Rostropovich is aware of this precedent, too.

Still, Rostropovich's baton technique is a matter of controversy among musicians, including the idea that his gestures at the podium are sometimes more those of a cellist than of a conductor. That suggestion was made recently by another conductor, who asked for anonymity. Some members of the orchestra call the suggestion "rubbish"; others find Rostropovich's beat sometimes hard to follow, but they quickly add that this is a common problem among conductors, and Rostropovich is "better than most."

"There is no bull---- in his conducting," says one brass player, "and there is in almost all of them. He conducts to the orchestra, not to the audience, without the flashy, meaningless, virtuoso moves you get from some conductors. He gives the music everything he's got -- 100 percent concentration all the time. He might get overexuberant, and then he can be hard to follow, but there's never any pretension." Others are less sure: "Sometimes you can see him enjoying the sound so much that you never know when he will let a note go and you can come in with the next phrase." In the first year or two, he "wasn't very good about communicating changes of tempo," according to one player, "but he has grown amazingly in this ability, showing the new tempo with the upbeat. He is also very conscientious about getting the right tempo at the beginning of a movement so that he won't have to change it during the actual performance. Sometimes you can see him, during the pause between two movements, singing the opening of the next movement to himself and finding the right beat with his hands."

Baton technique is probably the most subjective area of a conductor's art -- the textbook diagrams of what should be done with a baton are clear and precise, and no conductor worth hearing ever follows them clearly and precisely. Klaus Tennstedt, who wooed the NSO into some of the finest playing of its history a few months ago, sometimes looks (from the audience) like a gigantic and very musical stork taking wing.

Ultimately, judgments on baton technique boil down to the question of whether an orchestra can read the conductor's intentions. On this point, opinions vary intensely among NSO members; baton technique is probably the most hotly debated subject that comes up in informal conversations among orchestral musicians, and also the subject on which conductors are mostly harshly criticized. Quite a few NSO players say that they would like more clarity in Rostropovich's beat, but in performance his message usually seems to get across -- at least when he has carefully worked out his concepts and had enough time to present them thoroughly in rehearsal.

There is much more unanimity on the quality of his ear, which all the experts (orchestra members and composers alike) find amazing. He can pick out a single wrong note in the sound of a full orchestra playing at top volume, and sometimes he can single out the instrument that went wrong when several instruments are playing the same part. Composer Ezra Laderman tells of a piano run-through of the score of his Fifth Symphony (titled "Isaiah") which will have its world premiere this season under Rostropovich's baton: "I played one wrong note in a big, complicated chord, and after I finished, Slava pointed to that spot in the score and asked me, 'Which is right--the E that you wrote or the E-flat that you played?' "

ONE OF THE best-kept secrets in secret-ridden Washington is Mstislav Rostropovich's annual salary. Estimates range from $300,000, which is probably too low, to $400,000, which is probably too high. Whatever the figure, it will be reduced by about 7 percent this year; he is returning one week's salary to the orchestra as his personal gesture in response to the musicians' acceptance of a wage freeze. Is he worth it? Leonard Silverstein, the orchestra's president and Henry Fogel, its managing director, both answer with an emphatic "Yes." The salary is in the standard range for star conductors, and Rostropovich is giving the orchestra 16 weeks of his time -- twice as long as Sir Georg Solti will spend with the Chicago Symphony this year. Fogel insists that Rostropovich is selling himself to the orchestra for far below his value on a free market: "He could easily earn that much in 16 weeks of freelancing -- a lot more if he freelanced only with the cello."

One relevant point is that Rostropovich's concerts attract larger audiences. Another is the way he has influenced the orchestra's earned income, which has more than doubled since he became the music director. In 1976-77, the last year under Antal Dorati, it was $1.8 million. The still unaudited figure for 1981-82 is $3.8 million.

Beyond questions of interpretation and conducting technique, the largest question of all -- one that cannot be answered casually after hearing a few concerts -- is how well Rostropovich is filling the complex role of music director. The simplest answer is that of fellow conductor Fru hbeck, who became principal guest conductor specifically because Rostropovich asked him to and because it is hard to say no to Rostropovich. "I know of no other orchestra that is improving steadily every year as this one is," says Fru hbeck, who has conducted most of the world's great orchestras.

He gives the credit entirely to the music director: "If the orchestra were getting worse, I would not take the blame, and I will not take the credit for its improvement." He is too modest, of course; the time spent under his baton (six weeks this year) has been a period of great growth for the orchestra -- partly because he shows the players what they are capable of doing. But Rostropovich still deserves the greatest credit -- including the responsibility for Fru hbeck's influence.

On Tuesday night, associate conductor Hugh Wolff will hand over to Rostropovich the finest ensemble that has ever played under the name of the National Symphony Orchestra; recent concerts have showed that again and again. In the last five years, the orchestra has been strengthened enormously by the arrival of new players -- attracted to Washington largely by the magnetism of Rostropovich. And the veteran members play with an unprecedented spirit -- a pride and a sense of enlarged capabilities -- that derives largely from the personality of their music director: not only the prestige he shares with them but his skill, his energy, his all-out (even reckless) dedication to music. There is a feeling of unlimited potential for conductor and orchestra alike--a feeling that may be a shade optimistic, given the orchestra's economic problems, but one that is essential for real achievement. How long does it take to raise an orchestra from mediocrity to assured greatness? In the case of Rostropovich and the NSO, evidently it will take more than five years. But progress so far has been impressive.