On paper the concert by Montserrat Caballe' and Marilyn Horne that 3,700 excited listeners jammed Constitution Hall to hear on Saturday night looked almost too sensational to be true. Here were two singers of indisputable greatness -- superbly matched both in style and sound -- presenting a lengthy program of bel canto opera that was of uncompromising quality and of enormous difficulty.

Perhaps it was going to be just too much to expect; one felt a touch of doubt. Also, the ever-reliable Horne had avoided rehearsals because of a cold. And Caballe''s incomparably sumptuous soprano has always been difficult to control.

So it took a little time before it became clear what we were in for. This was a truly momentous musical evening -- an amazing three hours long -- of singing that wavered in quality from the merely fine to the truly unforgettable. The format was this: an aria by Caballe', an aria by Horne and then a duet by the two. This was repeated five times over, counting the encores -- with an extra duet thrown in for good measure.

In the opening round, Horne, as they say, hit the ground running. It was that brilliant showpiece from Handel's "Rinaldo" that she sang here with the Met several years ago, "Or la tromba," in which the mezzo engages in a duet -- a contest, as it were -- with the trumpet, no less, each trying to outsound the other in clarion command. At the end, my companion really summed it up when she remarked, "The trumpet was terrific, but Horne won."

Comparing Marilyn Horne to a trumpet, of course, describes only one of the many aspects of this virtually flawless voice. But in the brilliant runs and ornaments required of this virtuoso coloratura material, Horne has the precise articulation and seamless tonal presence of that instrument. Horne is almost certainly the world's most accomplished singer of this particular kind of music . She was brilliant all evening (another stunning example: that incredibly bold interval at the end of a cavatina from Rossini's "La gazza ladra").

Caballe' started a little more slowly, with "Piangero la sorte mia" from Handel's "Julius Caesar." She was being a little cautious, sort of feeling out her voice to see how it would be doing. One remembers the performance less for socko impact than for exquisite details, like that pianissimo trill at the end that one doubts another singer could have matched, given Caballe''s legendary command of soft high notes.

By the second half, though, to say that Caballe' had everything at hand -- absolutely everything -- is to put it mildly. She simply stunned the audience with an aria, "Sola son io," from a little-known Donizetti opera, "Sancia di Castiglia."

It is an almost Verdian recitative and aria in which the character is wrestling with herself about poisoning her child. As the moods kept changing from phrase to phrase, Caballe' let loose with that range of dynamics and tone that are hers at her greatest -- from a Callas-like edge to a huge resonance that was almost orchestral, and all projected with enormous dramatic intensity. The audience let loose at the end with the stormiest ovation of the evening; it was a sustained roar that sounded more like what you hear at RFK Stadium than applause in a concert hall.

From then on Caballe' could do no wrong. Another example: that almost unworldly soft high at the end of "Ah, mi lasciate" from Donizetti's "Adelia."

In the duets, the two singers consistently pulled off that hardest of tricks -- each singer being heard with clarity at all times. It was a matter of matching of the voices: Each is dark in timbre, each has enormous presence and size, yet each also has such vocal control that she can shape the most subtle contrasts of color. Each half ended with one of the great duets from Rossini's "Semiramide," which Horne and Caballe' have been singing together for years, leading to a rapport Saturday night that was matchless. The final encore, furthermore, was the most famous, and probably the finest of all soprano/mezzo duets: "Mira, o Norma" from Bellini's "Norma," gorgeously sung.

An able pickup orchestra was under the expert hands of Nicola Rescigno, who has the bel canto mode as much in his blood as any conductor I can think of.

A final word: In an interview last week Horne commented on what she called the "Sutherland-Horne-Caballe' musical thread" and its impact on bel canto performance in our time. Thanks to the Washington Performing Arts Society, we have now heard all three within two months in peak form. It is a remarkable achievement, on which WPAS lost money despite packed houses. WPAS is a first-rate organization that has not lost sight of the fact that it has not only needs, but also obligations.