Faced with its greatest challenge of the week at Wolf Trap, the Metropolitan Opera on Saturday afternoon gave a performance of Mozart's "Don Giovanni" that did real credit to the masterpiece.

Much of the strength that surged continuously through the music came from the superb conducting of Theo Alcantara. His sense of timing was flawless in each changing mood and scene even when his rapid tempos put some of the singers to heavy tests. It is to the credit of each member of the cast that the singing steadily rose to the high occasion, even from those who left certain kinds of vocalism to be desired.

With James Morris and Donald Gramm matching wits and musicianship as Don Giovanni and Leporello, and Allan Monk a vivid and vocally polished Masetto, the opera's men, abetted by John Macurdy's Commendatore, were very well handled indeed.

Morris moves, acts, and sings like a Don in the grand tradition. His voice is a touch heavier than the ideal sound if fine balance between him, Leporello, and the Commendatore is to be achieved, but he uses it like the rapier with which he is so adept.

Gramm has to be the top Leporello anywhere today. He realizes every nuance, note, syllable and dramatic expression of the part so ideally that it comes out sounding as if he had been born into the job.

The same must be said for Roberta Peters, whose Zerlina, from the time she made her debut in it 28 years ago, has remained not only one of her finest parts, but a model. She was a delight.

Joan Carden, new from Australia, sand Donna Anna with a fine voice which she frequently fails to control. Often too loud, she showed on the single word "abbastanza" what she can do when she wants to. Only polishing is needed to make her Anna notable.

Maralin Niska has problems as Donna Elvira. Her voice has become edgy and plagued with a fast vibrato that works against the good things she wants to do. Rockwell Blake, who recently made his Met debut, sang Don Ottavio very very vocally. He takes good care not to overdrive his smallish sound, and the musical results were admirable.

It was a shame that the program thanked Mrs. Edgar Tobin and Robert Tobin for their help in refurbishing the great Berman set, since Berman was nowhere to be seen in the unimpressive sets.