By the discreet protocol of the string quartet world, Saturday night's program by the Guarneri Quartet to a standing-room-only crowd at the Kennedy Center's Terrace Theater was a bit of an adventure.

The composers' names on the program were themselves a bit of a surprise, at least for this lustrous ensemble that tends, in its annual Kennedy Center concerts, to hew to the safe and rich center of the enormous quartet repertory.

This time, according to advance notices, the concert was supposed to end with Tchaikovsky -- and while Tchaikovsky chamber music is not exactly as much a household word as "Swan Lake," it is well within the bounds of quartet convention.

Instead, they hit us with Arensky -- Anton Arensky, an undervalued Tchaikovsky contemporary who wrote in much the same style, so western that it apparently eliminated him from membership, like his mentor, in the nationalistic Russian Five (Mussorgsky and company). This "Quartet in A Minor," Op. 35, was actually written as a memorial to Tchaikovsky after his death.

Why would one of music's leading string ensembles single out such a little-known, if appealing, work to end its program?

The reason soon became clear. Quite aside from Arensky's impeccable craftsmanship, the work is a flood of wonderful melodies. Some were as familiar to the ear as the work was unfamiliar to most of us. In the central set of variations (Movement 2 of three) the theme is taken from one of Tchaikovsky's "Children's Songs" called "Legend." It is a flowing, rhapsodic series of variations not unlike Tchaikovsky's own cello "Variations on a Rococo Theme," full of flourish and fire.

But the tune that really hit you was in the last movement, that great choral hymn -- one of the dozen or so landmarks of Russian music -- that is the core of the Coronation Scene from Mussorgsky's "Boris Godunov." Arensky takes it and treats it in all kinds of ways, including a vigorous fugatto. I enjoyed it so much that I hoped the Guarneri would repeat it when they indicated that an encore was coming. But instead the encore was a lovely Mendelssohn scherzo, from Op. 44, No. 3.

Another adventurous move was the performance of Barto'k's "Second Quartet," a bleak, intensely dramatic work from his early years that foretells what is to come in the most mature Barto'k, as in the "Concerto for Orchestra," which Antal Dorati was conducting, coincidentally, downstairs in the Concert Hall.

Over the years, the six Barto'k quartets have been almost the private preserve of the Juilliard Quartet, whose stark, angular playings of them have shaped how all of us conceive of these extraordinary pieces, probably the most substantial set of quartets since Beethoven. The Guarneri's performance was, predictably, a bit less driving and hard-edged than the Juilliard's -- but in the last movement especially there was a moving, ruminative dimension. Equally predictable, the players brought a wide range of color to the work: those perfectly meshed attacks and releases, those will-o'-the-wisp pizzicati (as in the last notes). Their sensitivity to the drama of the music makes one excited to think about what they will do with the other quartets.

The evening opened with a spirited version of Haydn's "G Major Quartet," Op. 54, one of those dozens of such works superbly balancing elegance and abandon that Haydn seemed without effort to sit down and dash off after his Wheaties in the morning. No other composer I can think of could have done it.