Two American orchestras brought notable concerts to the Kennedy Center over the weekend. Each program, led by a young American conductor, included unfamiliar music by two distinguished American composers.

On Saturday night it was the Buffale Philharmonic under the direction of Michael Tilson Thomas, with Clamma Dale as soprano soloist. After a reading of the Second Symphony by Borodin that had fine, romantic style and some dazzling playing, Thomas and Dale turned to Aaron Copland's settings of poems by Emily Dickinson.

The poems are familiar in the original version for voice and piano but Saturday was the first time the composer's version with chamber orchestra had been heard in Washington.

The songs, which are sensitive moldings of the subtle poems, were sung with probing feeling and lovely sound. Although there are times when the voice is covered by the instruments even with a smaller orchestra, the added instrumental color makes up for the minor problem of balance.

Thomas then added Carl Ruggles "Men and Mountains," pioneer music by one of our principal musical explorers. The performance wasin a rugged vein to match the writing, enriched by velvet string sounds in the "Lilacs" section.

Dale's account of Beethoven's "Ah, Perfido!" was brilliant, without penetrating much beneath the surface, and with some danger of overloading the big, high notes. A hard-driving reading of the suite from "The Miraculous Mandarin" closed the concert.

The Sunday afternoon concert by the St. Paul (Minn.) Chamber Orchestra was dedicated to the memory of the late Sen. Hubert Humphrey. It opened with a reading by Carl Eller, a defensive end for the Meninesota Vikings, of Wordsworth's poem, "Character of the Happy Warrior."

With Jorge Mester as guest conductor, the St. Paul Orchestra displayed superb form in a piece of unusual wit by Dominick Argento, his "Royal Invitation, or Homage to the Queen of Tonga."

The music, written in 1964, recalls the arrival of Queen Salota of Tonga in London at the time of the 1952 coronation of Queen Elizabeth II. With jolly references to "Rule, Britannia," which later become quite moving, Argento adds spicy wit and a fine array of musical delights in a score that ought to be a great popular hit, if only more conductors had the imagination to play it.

The late Wallingford Riegger is one of this country's best, neglected composers. Mester did him and us a rare service by presenting his "Study in Sonority for Ten Violins." And the 10 violins of the St. Paul orchestra did themselves proud with their expert, polished playing, a hallmark of the entire ensemble.

The Kennedy Center is too large for harpsichord solos, which meant that the effect of the Haydn Concerto, for all the excellent playing by Kipnis and the orchestra, was lost because the ensemble was forced continually to hold down its proper volume in order not to drown out the soloist.

Mozart's Symphony No. 29 in A closed the program, and again there was style and polish.