"When he's been up there for about half an hour, he will have shown his colors."

-Loren Kitt, first clarinet, National Symphony Orchestra.

THE NATIONAL Symphony had 35 guest conductors last season. Sometimes the conductor was in command. Sometimes the orchestra was in command.

All major orchestras engage guests to fill out their seasons, and the NSO's ranged from master musicians who got more out of the players then some realized was in them, to relative novices who needed help from them.

Talk to the members of the National about the most skilful of the guest conductors and two names time and again are mentioned: Erich Leinsdorf and Rafael Fruhbeck de Burgos. In fact, they are sometimes singled out at the expenses of the departing music director, Antal Dorati, and his successor, Mstislav Rostropovich.

Leinsdorf is 65; he is Austro-American; he is a specialist in the mainstream of the repertory; and he is gradually assuming the mantle of musical elder statesman.

Fruhbeck, the other master, is 44; he is Spanish; he leans toward French, Spanish and 20th-century repertory; and he is a man very much on the way up.

What these two conductors have in common is an uncommon skill at obtaining first-rate performances from orchestras with a minimum of fuss.

The NSO list of visiting conductors for 1977-78 isn't yet complete, but the policy for the future is plain - more big guns than ever before.

Leinsdorf is a precise, acute sort of man who hates to waste a minute. In his witty, spicy autobiography, "Cadenza," he marvels at the rehearsal techniques of Toscanini, with whom he worked before World War II, noting "the incredible efficiency of his preparatory work, a result of his lucid overall concept and his unwavering tempi."

Leinsdorf's desire that his time be well used is one of the reasons why, in a negative way, it was 39 years into a distinguished career on the American musical scene before he took the time, last April 5, to conduct a concert with the National Symphony.

If proof were needed, though, that conducting today's National Symphony is well worth the time of the Leinsdorfs of the world - or the Karajans, or the Soltis, or the Abbados, or the many other luminaries who have never stopped here except with other orchestras - Leinsdorf's spectacular performances provided it. In four short rehearsals, Leinsdorf had brought the players to an evenness in technical precision, tonal sheen and interpretive depth widely considered to exceed their level under any other baton this season.

Long before the end, the audience sensed that something very special had occurred, and gave prolonged ovations. The critics felt it, too. And most of all, the musicians: Veteran tympanist Fred Begun recalls the conducting of Brahms' Second Symphony. "It was a study in total command - of what conducting is all about."

The Leinsdorf appearance was another in the steps being taken to put the National Symphony in the international musical realm - a sort of jet-set world that is music's version of the lecture circuit. And if hiring Rostropovich was the most important of these steps, his ability to attract more and more of what managing director Oleg Lobanov calls "world-class" guest conductors will further upgrade the orchestra.

Lobanov acknowledges that Rostropovich, who has the last word on guest conductors, possesses a unique card to play in luring such guests: his willingness to perform solo cello with their orchestras in return. "There's no question that's factor in the equation," he says.

Leinsdorf hasn't yet been lined up for a return, though he says he has been asked. But this year Leonard Bernstein, with whom Rostropovich has recorded and who stayed with the orchestra during some its leaner years, will return. And the Boston Symphony's Seiji Ozawa, will whom the cellist has also made records, will debut. The following years will see increasingly large numbers of such big names, says Lobanov, even if the expenses means fewer soloists.

A greater emphasis on superior guest conductors can have a cumulative effect, because they play important roles in an orchestra's development. Guests provide variety and excitement, of course. But, most importantly, the experience for the orchestra of performing under a disciplined, demanding guest maestro becomes a test of an its mettle, and each time the players are asked to aim high, and make it, the odds improve that the next tim they will make it too.

Performances like Leinsdorf's here this spring do not happen by accident. They are the products of meticulous preparation on everybody's part and of a rapport that comes from shrewd mutual assessment by guest conductor and orchestrs alike of what each is capable of.

Leinsdorf gets a head start in this process by sending ahead of his arrival his own copies of each musician's parts, with interpretive details like bowings marked on them. These scores are from Leinsdorf's private library; he is one of the very few major conductors now to guest conduct exclusively, though in fact he is a kind of untitled principal guest conductor btoh at the Metropolitan Opera and with the New York Philharmonic. Leinsdorf resigned the prestigious music directorship of the Boston Symphony in the late 1960s out of frustration with the administrative responsibilities entailed in the job.

Leinsdorf approaches rehearsals with what he calls a standard formula. "You start with the longest work - in Washington it was the Brahms Second - and you play until you have an idea of what the orchestra already has and what you have to add. It's very convenient in a sonata form work like the Brahms first movement to play through the exposition and the development and then stop and give guidance when you get to the recapitulation (the point at which the initial themes are repeated). By that time you have a feeling of security on the part of the players and you know if you are going to have plenty of rehearsals."

Likewise, an orchestra will size up a conductor in a not dissimilar way. Kitt, the first clarinetist, observes, "With Leinsdorf we all realized his skilled rehearsal technique much sooner.It seems as if the mark of the really fine conductors is that they really have an idea of exactly what they want, and Leinsdorf has this. It's not so important that you agree with their interpretation as it is that they have a valid musical idea. Leinsdorf was not just guiding traffic. He was very specific and sometimes very blunt, no pussyfooting around. He sought out the problems and solved them. But he did it in a way that did not seem demeaning."

Leinsdorf's description of his conducting philosophy would seem to match Kitt's account: "It's essential that the conductor conduct for the orchestra and not for the galleries. A good orchestra wants to perform well. And it is important for the conductor to be consistent, so that they know what is going to happen.Otherwise they will get nervous. In particular, the conductor has to consider that he is playing with human beings, not instruments."

A characteristis of Leinsdorf's conducting is an unusually specific baton technique with detailed gestures on tempo, dynamics, tone color, note values and balance. He is very much the pragmatist on matters about which other conductors would be doctrinaire, such as seating arrangements. In the Brahms, when the trumpets and the tympani, on opposite sides of the stage, complained that the Concert Hall's acoustical vagaries made it hard for them to stay together, the conductor's solution was simply to move the brass across stage.

Fruhbeck is an increasingly frequent visitor. His moment of greatest challenge here came last month when he was responsible for successful preparation and performance within a span of only rwo weeks of 13 different, and difficult works, plus an evening of operatic arias.

Fruhbeck's rehearsal method was not unlike Leinsdorf's. He began by testing the degree of potential orchestral problems, beginning the first rehearsal with the most complex passage on any of the programs, the devilish final dance from Stravinsky's "Le Sacre de Printemps." He repeated it no fewer than 12 times, until he was satisfied, and then worked backward through the programs from there. "It was a fantasic feat of concentration and self-control," recalls Begun, whose tympani are at the pulsating epicenter of "Sacre's massive eruption of sound.

The arts of working with, and being worked by, guest conductors are like the problems of handling house guests. They can be the hit of the party, as with Leinsdorf and Fruhbeck. They can make the party a bore, as happened several times last season. And they can, in the style of "The Man Who Came to Dinner," come for a visit and end up taking over; This, in essence, is how Rostropovich came to replace Dorati a year earlier than Dorati had planned to be replaced.

The choice of guest conductors is dictated by what the music director and management perceive as the orchestra's needs. The New York Philharmonic, for instance, is in between music director this coming season and the leaders in its 32 weeks of subscription concerts are a rather dazzling nine guest conductors (with Leinsdorf opening the season) any one of whom would probably satisfy National Symphony manager Lobanov's definition of "world-class."

For different reasons, the Philadelphia Orchestra is bringing a wide selection of high-powered guests, all of them more or less on trial, in preparation for that inevitable day when a replacement must be found for Eugene Ormandy, now in his 42d year as music director.

Some other orchestras traditionally have depended less on guests.Leinsdorf decries the constraints on hiring guests that were imposed on him in Boston, "a holdover from the Koussevitsky days when he didn't like having them around."

Another constraint facing all orchestras is money, because "world-class" conductor do not come cheap. And budget comparisons explain why Washington can afford fewer than its New York and Philadelphia equivilents. Two years ago, the most recent season for which comparable figures could be obtained, New York was budgeted on the high side of $5 million, Philadelphis on the low side of $3 million, according to an authoritative source. That year the National's only "world-class" guest was Rostropovich.

Guests conductors' fees are closely guarded secrets, and will vary according to the circumstances. But as one expert on the subject notes, "no one has bothered to deny" a published report last year that at that time the highest standard fee was Herbert von Karajan's at "$6,000 plus."

Othes citied in the $6,000 range were Bernstrin, Sir George Solti and Karl Boehm. Leinsdorf and Ozawa were said to be in a middle range behind them.

Even the hiring of a "world-class" guest is no guarantee of "world-class" results.

There was a concert of Beethoven symphonies here in the mid-'60s that paired a musical giant, the late Otto Klemperer, with a superb orchestra, the Philadelphia. Klemperer's Beethoven recordings are legendary, and hopes were running justifiably high for that concert. But for some reason (reportedly Klemperer's unfamiliarity with the orchestra and his inflexibility at rehearsal) that communicative chemistry that makes for memorable performances never took fire. And the Beethoven came out in the last way one would have expected - dull.

Sometimes these failures develop into antagonism between the guest conductor and the orchestra. The most celebrated such event in recent years was the 1967 flap between the young Zubin Mehta and the New York Philharmonic. After some particularly stormy rehearsals, Mehta announced to the press that the Philharmonic was inferior to his own Los Angeles orchestra and that "They step over conductors. A lot of us think, why not send our worst enemy to the New York Philharmonic and finish him off once and for all," It looked like Mehta and the Philharmonic were finished.

But time makes even egos of this magnitude forget, and Mehta was invited back to the Philharmonic as a house guest in 1974. They must have made up, because Mehta has accepted an invitation to return to the Philharmonic next year not as a house guest - but as head of the household.