Last year's New York City programs were an exception, for Slatkin used those concerts to fine-tune standard works the orchestra was about to take on its European tour. This year, as if to make up for its conservatism in 1985, the Symphony will play two new works in Carnegie Hall. Both are by major American composers and are sure to attract the attention of important East Coast critics.

The April 12 New York concert will open with Joseph Schwantner's ''A Sudden Rainbow,'' an 18-minute tone-poem composed in Schwantner's third year as composer-in-residence here, and unveiled in Powell Hall Jan. 31. The April 11 concert will begin with Steve Reich's ''Three Movements for Orchestra.'' Reich's 20-minute score was completed only nine days ago and will have its first performance Thursday evening in St. Louis, at the Symphony's subscription concert.

THE REICH PIECE is of unusual importance in being a ''Minimalist'' composer's first major work specifically for large orchestra.

Reich has been represented in recent years on orchestral programs in this country and in Europe. His two most recent large-scale pieces, however, involve chorus as well as orchestra; the 1981 ''Tehillim'' is a four-movement setting of Hebrew psalms, and the 1984 ''The Desert Music'' is a cantata based on poems by William Carlos Williams. Both works have been recorded, on ECM 1215 and Nonesuch 9 79101-1, respectively.

Another orchestral item in Reich's burgeoning disco-graphy is the ''Variations for Winds, Strings and Keyboards'' (Philips 412 214-1), but this is a 1980 revision - commissioned by the San Francisco Symphony - of a piece originally written for chamber ensemble. Prior to the orchestral ''Variations,'' the most massive forces Reich addressed were the groups of soloists gathered for his 1976 ''Music for 18 Musicians'' (ECM 11and his 1978 ''Music for A Large Ensemble'' (ECM 1168).

IN AN INTERVIEW in the January, 1986, issue of Musical America, Reich noted that the mere act of scoring ''The Desert Music'' for full orchestra had a deep effect on him. ''What [it has done for me,'' he told K. Robert Schwarz, ''is to open up the orchestra and say: Now the orchestra is yours, go ahead and write anything you want for it. That confidence has been of central importance in writing the new piece for the St. Louis Symphony.''

Reich was born in New York City in 1936, and his musical training followed the norm for many young composers of that generation. In his high school years he played percussion instruments, but when he set off for Cornell University - at the age of 16 - it was to pursue a degree not in music but in philosophy. A music history professor at Cornell reminded Reich of his creative bent and talent.

After writing his bachelor's thesis (on Wittgenstein) he studied composition privately with Hall Overton, then at the Juilliard School of Music with William Bergsma and Vincent Persichetti, and then at Mills College in California with Luciano Berio and Darius Milhaud. Reich produced a lot of music in those days, most of it of the sparse, angular and intellectually fashionable sort now referred to as ''post-Webernian serialism.'' In 1963 he simultaneously formed his own percussion group and shook off the restrictions of ''formulaic'' academic music.

THE EARLIEST work by which Reich is generally known today is the ''Come Out'' (Odyssey 32 16 0160), a virtuoso tape-recorder composition in which a snippet of speech is recycled for 13 minutes, first with two channels of sound in precise unison, then with the channels slightly out of phase, then with the fragment divided into four and finally eight discrete unsynchronized voices. Eventually the music grows into a swirling mass of phonemes in which verbal content is entirely subordinated to sonority. More important, the method by which Reich shifts the phasing is readily apparent to the careful listener, and so the piece can be taken not just as a texturally rich study in musique concrète but also as a work in which process is in essence the composition itself.

The idea of music as process was articulated by Reich in a 1968 essay and demonstrated in most of the pieces he finished between 1965 and 1973. Some of these earlier works involve, as does ''Come Out,'' the manipulation of sounds pre-recorded on tape, and one of the pieces from 1969 makes use of an electronic device Reich invented and called the ''phase shifting pulse gate.'' All of them - including the 1971 ''Drumming'' (Deutsche Grammophon 2740 106) that has since become a sort of signature piece for Reich and his ensemble - are based on a gradual, systematic transformation of just a few very simple melodic and rhythmic elements.

In the mid- to late 1960s the American composers Philip Glass and Terry Riley were going about music-making in quite different ways. Glass was constructing long melismatic pieces by adding notes one at a time to melodic cells that might be given 20 or 30 reiterations before evolving to new stages of development; Riley was creating quasi-improvisational pieces in which the players worked their way through series of much-repeatedmelodic figures by moving ad libitum from one to the next. Glass' and Riley's music from that period shared several features with Reich's, among them harmonic stability, fairly steady pulse and emphasis on repetition. Their music also had in common the fact that, as complex as a piece might seem to become as it unfolded, its basic content was in a very real sense minimal.

LIKE GLASS AND RILEY, Reich began to withdraw from the Minimalist approach almost as soon as the adjective crept into the jargon of music journalists. His works from the late 1970s still used repetition and steady pulse, but the static harmonies of ''Drumming'' et al. were exchanged for chord progressions that lent the newer pieces genuine dynamic motion. Although they were built of long stretches interrupted by neither punctuation nor cadence, ''Music for 18 Musicians'' and ''Music for a Large Ensemble'' nevertheless offered plenty of suggestions that Reich had become interested in the flux of tension and relaxation associated with harmony throughout the 18th and 19th centuries.

Speculation that Reich was on the verge of a rapprochement with traditional harmonic structuring was confirmed by the 1981 ''Tehillim.'' It retains the rhythmic insistency that is Reich's trademark, but melodies are extended enough to be regarded as germinal themes, the tempos of the four movements more or less follow the pattern of the Classical symphony and - most significant - the sections of each movement are demarcated by points of harmonic climax. Similar observations can be made about ''The Desert Music,'' although in general the cantata makes reference to traditional music more with its expressive devices than with its architectural details.

IN HIS PROGRAM note for ''Three Movements for Orchestra,'' Reich points out that certain percussion instruments ''play constantly and supply the on-going rhythm of the piece,'' that in the second movement the music ''gradually moves from pulse to melodic patterns in such a way that it may be difficult to say when the pulses end and when the melodic patterns begin.'' He also points out how the string players are divided into two groups in order to clarify episodes of counterpoint, how the final section is constructed along the lines of a mensuration canon, how both the first and third movements resolve unambiguously in the key of A Minor.