"I'm a fairly good pianist, but it's not because I practice," composer Ned Rorem told last night's audience in the Kennedy Center Terrace Theater. "It's because I'm a fairly good mimic. When I sit down at the piano, I'm doing my imitation of Eugene Istomin."

It was Rorem night in the Terrace with three other artists joining in the tribute, the second in the series of American Portraits. More than 30 years in the creative career of the still youthful-appearing composer were represented in works that reached from his 1948 toccata for piano to a stunning tour de force for solo cello written last year.

Called "After Reading Shakespeare," the nine cello pieces were inspired by such passages as Lear's dying "Never, never, never, never, never"; and the entrances of Oberon and Titania: he with his train, she with hers. For each scene Rorem found music that touched the magical: mocking pizzicati to reveal Oberon, an elegant strain for the aloof Titania. An exquisite love song called up the ravishing beauty of Renee Asherson's Katharine opposite Laurence Olivier's Henry V.

Another lyrical passage, suggested by "The quality of mercy is not strained," could be a profile of the Portia soon to give judgment in the Supreme Court. There is not an uninspired moment in the entire set. Skittering harmonics taken at a fearful pace recall Sonnet 60, "Our Minuets Hasten to Their End"; an elegiac melody asks "Why Hear'st Thou Music Sadly?" from Sonnet 8. The final virtuoso allegro arises out of Iago's "Demand me nothing. What you know, you know."

Sharon Robinson, who commissioned the Shakespeare scenes, played them with a breath-taking mastery enhanced by an inner rapture in the quiet episodes.

The evening began with pianist Jerome Lowenthal playing the toccata, three etudes which Rorem wrote for Emanuel Ax, and the three Barcarolles written in 1949 and first played by Leon Fleisher. The composer's own remarkable gifts as a pianist, despite his disclaimer at the beginning of the evening, were clear in every piano phrase heard during the program. When he served as pianist in the final group of songs, with soprano Rosalind Rees, he demonstrated his own very special art.

If the piano is Rorem's instrument, song has been, throughout his career, the realm he has visited most often. A cycle for soprano, cello and piano, offered by Rees, Robinson and Lowenthal, to poems by Wallace Stevens, represent the composer a decade ago with subtle complexities of thought. Six songs from the 1950s glory in his gift of expressiveness in simplicity. "Such Beauty As Hurts to Behold" is music of splendor, while "Spring" is all laughter and delight. The gravest thoughts emerge in the setting of Elizabeth Bishop's "Visits to St. Elizabeths," written after the poet visited her friend, Ezra Pound, during his years there. It is a magnificent example of mounting fury to which Rees gave every ounce of meaning.