A battle has been raging in baroque music for some years, and Sir Neville Marriner, one of the world's leading baroque interpreters, has run up a white flag.

"They've won," says Marriner, who has been in town for two weeks -- his last performance is tonight -- conducting the National Symphony Orchestra in programs of emphatically nonbaroque music.

Marriner and his Academy of St. Martin-in-the-Fields orchestra have effectively been driven from the 18th-century repertoire that made them famous by the purist demands of the early-instrument movement, a development in music every bit as drastic as the invention of electronic instruments.

Marriner is remarkably sanguine about the revolution.

"I think the interesting thing about the Academy is that it has managed to survive," he says. "Eventually, we will become yet another orchestra in London, but it will be the best one."

In 1959 in London, Marriner founded the Academy of St. Martin-in-the-Fields, which became known in the 1960s and '70s as one of the world's leading baroque orchestras. At that time, the Academy fluctuated between 20 and 30 active members. Today, it usually brings 70 to 80 players onstage when it performs, drawing them from a pool of about 125 musicians who have no contracts but an understanding with the orchestra. They play such composers as Beethoven, Schubert and Mendelssohn -- not the baroque repertoire identified with their past history. Next year the Academy will record Beethoven's Ninth Symphony -- a long way from the little Vivaldi concertos so prominent in its past.

The movement that caused all this began a couple of decades ago in an effort to lend authenticity to pre-19th-century music by using the kind of instruments that were in use when the music was written. It started with harpsichords and recorders, then spread out to include the viola da gamba, the lute, valveless horns and high-pitched clarino trumpets. It has now carried the day. In the current orthodoxy of old music, violins, violas and cellos must be equipped with gut strings, and the A below middle C must be tuned to 415 cycles per second, not 440 as in modern orchestras, or any baroque composition played will be considered unacceptably corrupted by modernism. That may sound extreme, but it is now a pervasive reality in the commercial and artistic vineyards of classical music.

Now, having secured a solid beachhead in 18th-century music, the "period" instrument movement has surged onward to Beethoven and Schubert, with advance scouts probing for comparable technological incompatibilities in performances of Brahms, Dvorak, perhaps even Debussy and Stravinsky. Where it might end was hinted a few years ago by composer John Harbison. "A century from now," he said, "I don't want some pianist to be considered an expert on my music just because he owns a 1980s Steinway."

Marriner, for all his hard-won reputation in baroque music, takes a moderate position. "I think they've won quite rightly," he says. "Because the critics have influenced public taste enough, the public really doesn't want to hear Vivaldi, Handel, Bach and Telemann played on modern instruments any more.

"I hated original instruments 20 years ago; I thought they were crummy then, because the performers were so indifferent." The Academy experimented privately with period instruments in those days, "and they could play them well enough. The thing was, they had to make a living, and a lot of them had string quartets or played in symphony orchestras, and they had to play modern instruments there."

After a while, people began to get the feeling that old instruments automatically meant authentic performances. Whenever the Academy produced a concert or a record, Marriner says, people would say, " 'Yes, it's fine, it sounds just like the Academy, but what a pity they don't play on original instruments.'The Academy decided, 'To hell with this,' " he says. "We decided to drop that sort of repertoire or give away as much of it as we could. We moved on to Beethoven, Schubert and Mendelssohn. Suddenly you find yourself in the middle of the 19th century or in the late 19th century, and you're becoming a much, much larger orchestra. This is what happened to us."

"Strangely enough," he has discovered, "it is not that much harder to administer an orchestra of 70 or 80 people than it is an orchestra of 20 or 30. Just a few more telephone calls to make."

Expenses are higher, naturally, but so is income, and with a bit of hustling, it seems to be balancing out. The orchestra has been given first choice of evenings in London's Royal Festival Hall throughout the season, and its fees for tours and recordings have been raised without any complaints from the presenters or recording companies. "When we go on tour, as we did a couple of weeks ago in Germany," Marriner says, "the promoters pay everything: the musicians, the hotels, transportation, all the additional costs, and the orchestra automatically asks for a 12 percent administrative cost {to help meet its nontour, day-to-day costs} on top of that. Well, 12 percent for an orchestra of 70 players is enormously more than for an orchestra of 20 players. That has sorted itself out very satisfactorily. The same with making gramophone records. When we made records with 25 players, we got an administrative fee on top of the costs. Now that we make them with 70 or 80 players, we still get the percentage, which the record company seems happy to pay, and it is much more."

Still, the orchestra -- along with every other orchestra in London -- is finding money tighter than it used to be, Marriner says. Recently, the Academy has begun to seek corporate sponsors, something it has never done before. "We never needed it," Marriner says. "The orchestra always made enough money -- in fact, showed a small profit and gave money away at the end of the year. We had a remarkable 25-year record of not having any sponsorship at all and still making a profit. But now that we are invited to have first pickings of the {very large} Festival Hall, we suddenly come up against the grim reality that unless you give the audiences absolutely ultraconservative programs you're going to lose on every concert."

Now, the Academy of St. Martin-in-the-Fields is facing the kind of problems Marriner used to face in his past positions with the Minnesota Orchestra or the Los Angeles Chamber Orchestra. "The name of Hindemith on a program can lose as many ticket sales as a snowstorm," he says, shaking his head. "And even the name of Tchaikovsky won't automatically sell tickets; it has to be a Tchaikovsky piece that lots of people know and like."

But in spite of these restrictions, Marriner has no wish to retreat to the 18th century and valveless trumpets. The Academy is content in its new manifestation, he says, and as director of the Stuttgart Radio Orchestra, Marriner can program a large number of contemporary works every season.

You don't have to sell tickets on the radio, he explains.