Just as things began to look hopelessly grim at the Metropolitan Opera - stodgy revival after stodgy revival with lackluster casting - Wagner's ''Tristan und Isolde'' and Beethoven's ''Fidelio'' came along as reminders that the Met still can and does offer the best available today. Neither one boasted an ideal cast, but what made both evenings good set them apart from what had preceded them to date.
Conducting sparked the revival of ''Tristan.'' James Levine, who had been heard unfavorably in the opera a few seasons back, refined, toned down, and re-thought his approach to the work. It may have lacked the beguiling allure of the greatest readings, but if all the Wagner heard today were of the substance Levine brought to this ''Tristan,'' we would have little cause for complaint.
In the climactic moments in particular, Levine's performance coalesced without ever prolongedly blanketing his singers. In the quieter moments of each act, he found the right mood and dynamic to make the most of the orchestral textures and to be fair to his singers.
Hildegard Behrens, singing Isolde for the first time at the Met, lacked the heroic timbre and cutting power to be ideal. Nonetheless, in today's small-voiced world, she can hold her own, can save shrewdly for the big moments, and create a histrionic/vocal aura that allows us to accept this approach to the role. The soprano remains a vivid actress, and she brought the role impressively to life, without ever erasing memories of Isoldes past.
Richard Cassilly has the vocal heft for Tristan but not the stamina these days. Nevertheless, he performed impressively, singing with force, declaiming with passion, and, when unfurling the braying tenor to its fullest, showing just what a big voice means in the Met. Miss Behrens's soprano shrank instantly by comparison.
Tatiana Troyanos, the Brangane, began weakly, but in the second act sang with great tonal beauty. Mr. Haugland made an aurally distressing Mark, Richard J. Clark a squally Kurwenal. The rest of the smaller casting was undistinguished.
As for the production itself, the lighting has been improved so that one cannot see all the elevators and other tricks that give this staging the illusion of infinity. Director August Everding's vision in this opera is one of cosmic bliss and oblivion - each time the legendary lovers are together, the world around them dissolves into starry, galactic night. There are fewer galactic manifestations now than when the production was new, but the sense of a great production (it was designed by Gunther Schneider-Siemssen) remains intact, which is presumably because of Mr. Everding's being on hand to oversee the revival.
''Fidelio'' featured Eva Marton in the title role and the debut of Klaus Tennstedt in American operatic conducting. Miss Marton, fresh from her triumph as ''Turandot'' in Boston, gave the role splendid amplitude of voice and an increasing commitment to the unfoldment of the character's dilemma. The voice is big, cutting, and thrilling when unfurled in all its majesty. Nonetheless, she lacked the sense of inhabiting the role and making Leonore, disguised as Fidelio to save her incarcerated husband, come totally, warmly to life.
In Matti Salminen the production offered a Rocco of remarkable distinction. He acted superbly and sang with effortless rolling tones and a vital sense of characterization. From there on it was downhill vocally. There was the honorable but uninteresting Florestan of Edward Sooter (substituting for the indisposed Jon Vickers); the uningratiating Don Fernando of Aage Haugland; Franz Mazura's low-key and not especially well sung Pizzaro. Roberta Peters's Marzellina was marred by excessive cuteness and dry vocalism, while Michael Best made a pallid Jacquino. The production looked rather threadbare, and the staging lacked impetus.
In the pit, Mr. Tennstedt gave forth with a noble, propulsive, electrifying performance of the score. His roots are clearly in the opera house. The Met orchestra responded to him the way it does to few other conductors these days, playing like the important ensemble it has become these past few years.
Tennstedt offered the score in his own way - now rather slow, now exceedingly peppy - but he managed to remain true to the spirit of the work. He made the ''Leonore No. 3'' overture between the two scenes of the second act sound as if it really belonged there, and the finale was of an ecstatic order.