Pinchas Zukerman is not exactly putting down his baton, though the St. Paul Chamber Orchestra is looking for a new music director to take his place in June 1987. What Zukerman decided last week, according to the orchestra's announcement, is that "he prefers making music to making administrative decisions." As Zukerman put it, "I wish to continue my musical association with the SPCO but am relinquishing my title as music director." A few days later, he politely declined to discuss the subject further.

It is likely that we will still see him conducting after the separation becomes formal. He does not plan to take on another orchestra, but he has already volunteered to conduct the SPCO on tour in the season after his retirement. There is a lot of bother in the job of music director -- budgets and schedules and auditions -- but once the conducting bug is in your bloodstream, it is hard to get it out.

It is also hard to balance a solo career with the life of a conductor, though more and more musicians are trying. A partial list of those who have gone that route in recent years would include Daniel Barenboim, Vladimir Ashkenazy, Jaime Laredo, Heinz Holliger, Jean-Pierre Rampal -- even such singers as Placido Domingo and Dietrich Fischer-Dieskau. And of course, as no Washingtonian needs to be reminded, Mstislav Rostropovich.

Why do they do it? In most cases, the answer is certainly not financial. A well-known soloist can easily earn more in a single night of performing his specialty than a conductor earns in a week -- which includes three or four concerts and long hours of rehearsal. An educated guess is that Rostropovich receives at least three times as much for two hours of cello music as he does for a week of orchestral concerts. Other soloists do even better.

Rumor has it that Vladimir Horowitz took home more than $60,000 from his last performance in Washington, and that Luciano Pavarotti has cost more than $100,000 for some one-night stands. Liberace's gross from his last visit to Washington was reportedly closer to half a million -- but, of course, he stayed longer. On a per diem basis, Horowitz is probably the highest paid pianist who has ever performed in Washington. In any case, conductors aren't in the same league economically -- when they are conducting.

So what makes the Rampals, Zukermans and Domingos of the world look enviously at the Soltis or Bernsteins? In the case of singers, the most obvious answer is longevity; voices fade away while orchestras just go on playing. A similar explanation will do for Gerard Schwarz, who has made a successful transition from trumpet virtuoso to conductor; wind players tend to run out of breath as the years accumulate. But that answer will not do for most instrumentalists. Some, like Casals, Segovia and Rubinstein, continue performing into their eighties or nineties.

The most obvious answer is power. A conductor twitches a little stick and produces sounds that no single instrument can approach. He makes a little gesture with his left hand and a half-dozen brass instruments blare out fortissimo chords. There is a glory in it that no piano, violin or cello can match.

There is also the question of variety. A star instrumental soloist can master a dozen concertos, a handful of sonatas and a few little encore pieces or novelties and take them around the world, year after year. In classical music, most members of the audience (particularly those who flock to celebrity concerts) prefer to hear this year what they heard last year. This is fine for a performer who doesn't want to work too hard, but it can become maddening for someone with an inquisitive mind or a low boredom threshold who is well-paid to perform the "Moonlight" or the "Kreutzer" Sonata over and over again. After the 200th "Appassionata" or "Emperor" Concerto, some soloists begin to wonder whether there aren't other worlds to conquer.

The vast and enormously varied orchestral repertoire is the most obvious answer. Material for the piano comes closest to the richness of orchestral music; certainly there is more of it than any player can master in a single lifetime. But for other soloists, the possibilities are much more limited -- particularly when you take account of quality, variety and what audiences are willing to pay for. A violinist can learn one new Baroque concerto per week through his whole career without running out of material. But there aren't many real masterpieces, and after a while, he might have trouble telling the more routine works apart.

The shortage of first-class material is even more acute for such instruments as the trumpet, flute, oboe or cello. Rostropovich has known all the significant cello repertoire for decades and has enlarged it considerably with new commissioned works. Apparently, that was not enough.

But there is a dark side in the world of conducting. With the power of the baton comes a proportionate responsibility. The soloist needs to worry only about his own instrument; the conductor is concerned with 100 or more, individually and collectively. If someone is out of tune in the second violin section, the conductor is to blame; if the woodwinds don't mesh perfectly with the strings, the man on the podium is the one who looks bad.

This is true, whether the conductor is a guest or the resident maestro. For the conductor who is also a music director, one title really covers two jobs. The backstage work -- planning seasons several years ahead, trying to find guest artists who will be congenial to work with and attract audiences, replacing orchestra members who retire, staying within budgets -- can be as demanding as the purely musical functions.

This assignment is evidently what made Zukerman decide to leave St. Paul.

The vastness and variety of the orchestral repertoire also means that a conductor has a lot of learning to do. Those who turn down music director positions and function only as guest conductors can survive with a few specialties, like star soloists, using the same material in London one week and New York the next. The resident conductor enjoys no such luxury; he faces the same audience every week, and normally he must allow years to pass before he can bring back a work he has already conducted.

Every conductor gets his start in music doing something else; even Lorin Maazel, who began conducting at age 9, started at 5 on violin and piano. Most conductors seem to begin as pianists -- Previn, Ashkenazy and Bernstein to name a few who are currently active; Bruno Walter and George Szell from the last generation. Zubin Mehta and Arthur Fiedler began as violinists, Leopold Stokowski as an organist, Serge Koussevitzky as a double-bass virtuoso, Arturo Toscanini as a cellist.

But getting started on an instrument is not the same as achieving an international reputation. Most noted conductors of the past seem to have been proficient on one or more instruments, but few achieved the kind of solo celebrity enjoyed by Zukerman, Ashkenazy or Rostropovich. It may be that the more leisurely pace of international concert life in earlier days did not make star soloists so nostalgic for the relative stability of a music director's life. Compared with the soloist who may be in Tokyo one day and Stockholm the next, a music director spends a significant part of his year in one city, going to work at the same place with the same people each day.

When a famous soloist starts a second career as a conductor, the informed public reaction always seems to begin with interested skepticism and evolves into a long period of probation; credits from one field do not transfer easily into the other. In the case of Andre Previn, who began not only as a pianist but as a musician identified with jazz and movie sound tracks as well as the classics, the early skepticism was particularly intense. But he has since reached widespread acceptance, as have Ashkenazy, Schwarz and Zukerman. For Rostropovich, with many observers, the probation period is not quite over, but the marks are significantly higher each year.

Those who manage to combine the life of a music director with that of a traveling virtuoso may have the best of both worlds. But in the case of Pinchas Zukerman, this double workload, with the added responsibility of routine administrative work, must have been too much of a good thing.