Last night was a triumph for Alberto Ginastera in the Terrace Theater's continuing American Composer Portrait series. As always when this composer has visited Washington with music not previously heard here, the marks of genius were heard from the stage.

Two works made profound impressions, both written for the composer's cellist-wife, Aurora Natola-Ginastera. The first of these, a sonata for cello and piano, is a stellar new work for artists able to handle its staggering demands. Its four movements include brilliant allegros filled with technical hazards of breathtaking virtuosity. A composer whose wife is the cellist knows minutely the capabilities of the instrument and the player. But should a husband ask such diablerie from his wife? Last night it was clear that Aurora Ginastera revels in the difficulties set before her. The more that was asked, the more she overcame every hurdle.

In the slow movement, a kind of recitative and aria, her cello loosed a plangent melody in a flood of golden tone, whose final high note hung in the air like a far-distant star. At every point, she was superbly partnered by the phenomenal playing of pianist Enrique Ricci. In the slow movement, with magical pedaling, he created marvels in suspended clusters of sound. In the scherzo, a prestissimo taken with mercurial lightness, he was like Horowitz in Moszkowski: not to be believed.

The sonata is another monument to its composer's singular originality of thought, music rooted in great traditions, that explores new worlds and conquers them.

A Serenade for cello, baritone and chamber ensemble of nine instrumentalists was the second triumph of the evening. Love poems by Pablo Neruda are recited or sung over a fabric of fragile but ensorcellating strength. Solo oboe, bassoon, flute, clarinet, horn, double bass, harp and many percussion instruments surround the voice of the solo cello as it presents a string of cadenzas that divide the other solo voices.

There are the sounds of fingernails brushing over a large gong; a whisk sweeping across harp strings; mysterious susurrations that mirror the night and the moon and the stars and the naked body of the beloved. There are reminiscences of Ginastera's great opera, "Bomarzo," and sudden explosions of sound that recall the violence of "Beatrix Cenci." Baritone Carlos Chausson joined the Theater Chamber Players whom Leon Fleisher conducted with sensitive authority.

The concert began with the Third String Quartet to which Ginastera, in admiration of Schoenberg's Second Quartet, added a solo soprano. The Washington Quartet played, and soprano Jane Bryden deserves high credit for learning the difficult music in three days, following the withdrawal of a previously engaged artist. But the quartet cannot be fairly judged without a singer who has been able to live longer with its special requirements.