In Karol Szymanowski's opera "King Roger," which had its second American production last night at Wolf Trap, the forces of reason, moderation and tradition encounter those of ecstatic, pagan mysticism. Reason (embodied in King Roger, the title role) emerges from the experience severely shaken but determined to carry on. The encounter is presented in music of enormous power, by a composer who has yet to receive full justice outside his native Poland. Last night, he was well-treated by the performers, if not by the sound system.

Now approaching his centennial (the actual aniversary will be Oct. 6, and he died in 1937), Szymanowski has reached large audiences in America only through a violin concerto and a few piano pieces that used to be played by Arthur Rubinstein, who also gives the composer a strong supporting role in his memoirs. Rubinstein went to Warsaw in 1965 to see a production of "King Roger," and the capsule review in "My Many Years" indicates that he could have been a pretty good music critic if he had not become hung up on piano-playing:

"It sounded rather like an oratorio, the action was somewhat static, but the music was, throughout, noble and moving. The chorus had a prominent role and sang to perfection."

Rubinstein's remarks apply precisely to what was heard at Wolf Trap last night, including his praise for the chorus. In this production, it was the University of Maryland Chorus, superbly rehearsed and at the top of its form, which had some great music and made the most of it. The resemblance to an oratorio was accented last night by the fact that it was a concert, rather than a staged performance. Among the solo singers, tenor Richard Cassily had the most demanding assignment, as the mysterious young shepherd from the East, who wanders into Sicily and shatters King Roger's world with his preaching of a new religion of joy and love: "My god is young and fair as I . . . With ivy crowned, he bears a cluster of the vine." He sang impressively throughout and with particularly fine nuance in Act III, but his voice seemed to harden above mezzo-forte, where it spent much of the time.

In the title role, baritone Allan Monk maintained a steady, high level of performance throughout. Soprano Jeryl Metz, as the king's wife, had a few moments of glory to which she rose splendidly, and tenor George Grey filled well the modest demands of his role as the king's adviser. All four of these voices sounded better in the final act than they had earlier, either because the singers were finally coming to terms with the sound system or, more likely, because the sound engineer had finally mastered the special problems of this production.

It was an enormous production, with the choral and solo singers filling the stage and a large orchestra occupying what would normally have been the front rows of the audience, and it must have been a nightmare to install and monitor microphones effectively. As it turned out, the tone came through richly and usually in good balance (though sometimes the chorus swamped the solo singers and occasionally an odd instrumental detail would pop into prominence). But the diction, until very near the end, was so unclear that the opera might as well have been sung in Polish rather than a carefully prepared English translation.

This sort of problem is common on opening nights, but last night's opening was also the only performance. As far as could be determined in the circumstances, Richard Woitach, music director of the Wolf Trap Company, conducted a sensitive and eloquent interpretation.

Szymanowski composed "King Roger" at a critical period in his life and seems to have reflected the personal crisis in his music. His home was destroyed during World War I, and he went into a two-year period of musical silence, beginning in 1917, after which "King Roger" was his first new composition. In this opera, which was clearly enriched by his scholarly work as a student of history, philosophy and mysticism, he seems to be trying (in a very rarefied, mystical way) to come to terms with that cataclysmic experience. He does it by focusing his personal emotions on an earlier crisis point in human history: 12th-century Sicily, the crossroads of the Mediterranean at a time when forces were being set in motion that would ultimately burst forth in the Renaissance.

King Roger II of Sicily (1095-1154), whom Szymanowski encountered in his avocation as a scholar, is a historic figure tossed into a fantasy-like King Rene' in Tchaikovsky's "Iolanta," which was performed at the Kennedy Center a few months ago. A Norman conqueror, carrying on in the tradition of his ancestors, he created a new kingdom for himself in Sicily and southern Italy and established it as a strong sea power and a center of the silk industry. By all accounts a firm and enlightened ruler, Roger provides a good symbolic figure for one pole of the opera's cosmic conflict. The other pole, the mysterious shepherd boy, wins over the king's court and his wife, and finally reveals himself as the god Dionysus. After a dark night of the soul, the king greets the rising sun (and ends the opera) with a note of rather ambiguous optimism.

Musically, "King Roger" sounds just a bit conservative for a piece composed in the decade after the "Rite of Spring," (which Szymanowski was inclined to admire but not to imitate). Musical influences audible in the score range all the way from ancient Byzantine chant to Debussy's "Pelle'as et Me'lisande"; in fact, "Pelle'as et Scriabin" would do as a short descriptive title for the music. One effect of "King Roger" should be to make audiences curious for more music by this composer, who sounds modern but not too modern, who writes in a supple, elegant and expressive style of eclectic modernism, and who obviously had a wealth of things to say.

It may be that recording rather than staged production is the logical medium for this intense drama of ideas. There is one recording issued in Poland about a decade ago that gives a fair account of the score, but it takes some searching to track it down. A new recording, perhaps by Charles Mackerras after he finishes his cycle of Janacek operas, would be worth hearing.