On Saturday afternoon, John Mauceri will conduct the season's final performance of "The Barber of Seville" and lay down his baton as music director of the Washington Opera. But he is not fading away; he is committed to conducting assignments with the company for the next two years, and the Kennedy Center is about to announce his appointment as its music consultant -- a newly established position. He also will make his debut tonight as announcer, program annotator and commentator on a new classical music series for CBS cable television.

What happened at the opera? The decision on Mauceri's resignation was made last spring, but the announcement was postponed. Now, with the Kennedy Center about to make its announcement, there is a danger that Roger Stevens may seem to be stealing Mauceri away from Martin Feinstein. Not so.

Those who know the personalities involved assumed that disagreements between Mauceri and Feinstein were behind the recent announcement of Mauceri's resignation as music director. The most persistent rumor (which is true but not the whole truth) was that Feinstein refused to give Mauceri some conducting assignments he wanted, such as next season's (still unannounced) "Falstaff." But that was not crucial. When Mauceri was appointed, Feinstein had made it clear that as general director he would make the final decision on conducting assignments.

The real problem was a whole complex of decisions on a variety of large and small issues: long-range planning, the balance of repertoire for a given season, the choice of performing editions for various operas, instrumentation and the size of choruses, to name only a few. Mauceri, a self-confessed purist, sums it up: "I can be a pain in the --- in musical matters; I understand that . . . But I was unhappy making decisions and finding later that the decisions had been reversed. Martin and I are both very passionate men. Our disagreements were passionate, and the easiest way to resolve the tension as to who had the ultimate responsibility for musical decisions was for me to become a guest conductor."

"We agreed to disagree," is Feinstein's summary of the situation. "Each of us respects and admires the other's talents, and we plan to continue the relationship."

The two are currently discussing 1985 conducting assignments in an amicable way.