Mozart's ineffable "Cosi fan tutte" is one of the most sophisticated challenges in opera -- to everyone: the singers, the conductor, the orchestra and the designers. If its silly little conceit of a tale is to take on its true dimensions, as one of the supreme philosophical comedies ever penned, "Cosi" must flow like quicksilver. And that's exactly how fine the Metropolitan Opera's "Cosi" was last night at the Kennedy Center.

This marvelous performance illustrated once again the dictum that there's nothing wrong with the Met's much-maligned tour if a production goes out with the same cast and conductor who made it a hit at Lincoln Center.

Obviously, this "Cosi" had been prepared with great care for the New York season -- the quality of the ensemble work was truly superb -- and in the repeats there and on the tour, hitches have been eliminated. "Cosi" is a long opera, but this version unfolded with such grace and ease that the hour-and-45-minute opening act seemed like about 40 minutes to me.

The production, directed by Graziella Sciutti and designed by Hayden Griffin, is good (it's two years old). The conducting of Jeffrey Tate is truly outstanding, especially the steadiness of his pacing, which is crucial to the tricky coordination of an ensemble opera like "Cosi."

But grandest of all last night were the singers. All of them. There are six roles in this elusive masterpiece. Each performer sang with consistent beauty of sound (except for some understandable fatigue near the end) and great care in phrasing -- in rhythm, diction, coloring, dynamics and intensity. And acting. Overstatement is a no-no in "Cosi" and they all know it.

Each was aware that to work, "Cosi" above all, must be suave and elegant. This is at the core of its philosophical irony -- the very large point of this little fable of amorous devotion and deception, that things are not what they seem. Mozart's very strategy in "Cosi" is to juxtapose the perfectly poised choreography of his music with the anything-but-poised action on the stage to illustrate his own quite exalted notion of the relation of illusion to reality.

One hardly knows with whom to start in such an effort. But the bravura role is the sister Fiordiligi, if for no other reason than that she has some of Mozart's most taxing music. One really doubts that anyone else is singing Fiordiligi better than Carol Vaness these days. The music, with its harrowing intervals and incredible runs, lies almost perfectly for her. It comes so easily that she is left free to color this extraordinary stuff with great imagination. Furthermore, she has the determined feminity of the character delectably in hand. This production was designed originally with Kiri te Kanawa in the part. She was bland by comparison.

Diane Kesling was a comic delight as the other, more empty-headed, sister, Dorabella -- utterly at ease in the part and in the music.

The two suitors are often sung more weakly than the sisters. But from the opening trio, both David Rendall, as Ferrando, and Brian Schexnayder, as Guglielmo, were singing with great style. Those tricky moments of articulation in the opening sections were sung precisely on the beat and in tempo -- a rare occurrence. And both men were pouring on tone where that is often not the custom.

Betsy Norden's Despina was a madcap delight. And casting a singer of Cornell MacNeil's fame as Don Alfonso was an unconventional inspiration. The Don is the smallest part, but it is also the pivotal one. MacNeil lavished on it just as much vocal and acting finesse as he gives to something as major as Germont in "Traviata." It gives this Don Alfonso a kind of weight that makes "Cosi" all the more credible.

Tate deserves much of the credit for the coordination of the singers. Sometimes he seemed to be conducting them more than the orchestra, though the playing was impeccable except for an occasionally errant horn. In sonority he sees the score a bit more dryly and clinically that did Daniel Barenboim in the Washington Opera's classic "Cosi."

Likewise, Jean Pierre Ponnelle's more daring staging for the Washington Opera of the work has a sort of startling clarity that I prefer. The Met production is distractingly busy by comparison. But the New York sets certainly are beautiful.

A final note: It is hard to understand why the only repeat in this week's performances will be a badly received "Lohengrin," when the company has along a "Cosi" of this quality in a year when Mozart is magic at the box office