Masterpieces are commonplace at the Kennedy Center; you can catch a dozen or more in a good week. Sometimes (as with the Dvora'k Cello Concerto between tomorrow and next Monday night) you can catch the same masterpiece in two different interpretations. But Saturday night's masterpiece was something special: Handel's "Hercules," known to specialists but neglected by performers and audiences for almost 2 1/2 centuries, made its first appearance at the Kennedy Center--as far as anyone can determine, its first in Washington. It has some of the greatest operatic music ever set to an English text, and it should not wait so long for its next appearance.

"Hercules" was given, oratorio-style, in a single concert performance, launching the Kennedy Center's 1983 Handel Festival, which will continue with "Theodora" in April and "Xerxes" in May.

Perhaps the best among many impressive performances in this production was that of the Handel Festival Chorus, prepared by Norman Scribner. In "Hercules," Handel used his chorus to heighten and clarify issues and emotions--commenting, reacting and advising the characters in a style remarkably close to that of Greek tragedy. On Saturday night the chorus showed what technically brilliant, emotionally rich and varied music Handel composed for this purpose. Its tone was rich and well balanced, its diction crystal-clear, and its emotional involvement as intense as that of any soloist.

Morley Meredith, in the title role, faced two challenges and met them triumphantly. Dramatically, he must make several convincing changes: from a bluff, hearty conquering hero in Act I to an innocent husband puzzled by his wife's insane jealousy in Act II to a giant--almost a force of nature--suffering a horrible death in Act III. Musically, the role includes one of the most challenging arias in the bass repertoire--the death song "I rage," which combines intense dramatic expression with the kind of vocal ornamentation that later composers have usually entrusted only to coloratura sopranos. He was simply magnificent in all departments.

In the equally demanding role of Hercules' wife, Dejanira, mezzosoprano Susanne Marsee took a while to warm up fully (not helped by her relatively colorless music in the rather slow-moving first act), but she rose to the full height of her larger-than-life role in the jealous confrontations of Act II and the dazzling remorse aria of Act III.

Also specially satisfying in a long and demanding role was soprano Lorna Haywood as the princess Iole, the captive whose beauty inspires Dejanira's fatal jealousy. The keynote of the role is sweet innocence, which is almost impossible to convey effectively to a modern audience. She did it brilliantly, with special distinction in her first great aria, mourning the death of her father.

The orchestra had quite a few new faces (many of its regular members were busy playing for the Washington Opera), but there was a nucleus of first-class regulars (concertmaster Jody Gatwood, cellist Evelyn Elsing and harpsichordist J. Reilly Lewis) to hold things together. Under the able baton of Stephen Simon, there were few mishaps, and the production was well paced and balanced.