The performances of Handel's Messiah at the Kennedy Center this week, conducted without cuts by Robert Shaw, come very close to the ideal for this masterpiece. The playing of the moderate-sized National Symphony is expert, and the singing from Norman Scribner's Choral Arts Society is phenomenal.

An uncut Messiah is a rather infrequent occurrence these days. That Shaw ends the work around 11:10 p.m., after an 8:30 start, says much about the way he views the grand score.There have been far too many years when performances of Messiah, even with cuts, have not been over by 11:30, thanks to the layers of holy-gopiety that so often obscure Handel's finest inspirations.

But the secret, if such it is, of Shaw's triumph is to superficial matter of speed. It is rather the result of a lifetime of affectionate admiration for Messiah which has resulted in superbly planned directing of each vital element.

The orchestra, of around 36 players including organ and harpsichord, is frequently reduced to chamber proportions, especially in accompanying the solos. The resulting clarity in texture is a delight.

So is the work of the 100-voice chorus which sings in a light, clear tone reminiscent of a baroque organ operating on low wind pressure. There is available a seemingly endless reserve of sound for the larger moments, but these are always kept within the framework of the whole. The dexterity with which the singers respond to Shaw's subtlest wishes in dotted rhythms and precise execution of rapid florid runs is a fine achievement.

Shaw, who will be conducting Messiah again tonight and tomorrow afternoon, does not use double dotting in the overture. But the sense of style is as acute here as at all other moments in the large score.

One of the most effective innovations in the performances is the way in which Shaw has each episode, solo or choral, prepared in advance so that there is not the customary stopping and starting when soloists stand or sit for their next number. By this simple device, Shaw enhances the feeling of unbroken drama that binds the music together.

To this must be added his gift for maintaining throughout the course of the long oratorio an unmistakable kind of springy vitality, whether in an adagio or a fleet presto.

No success in Messiah is possible without a quartet of singers capable of mastering Handel's style as well as his technical hurdles. Contributing strongly to what is altogether the most satisfying Messiah in a long memory are soprano Lorna Haywood, contralto Florence Kopleff, tenor Gene Tucker and bass Paul Plishka.

Kopleff was overwhelming in the effect she created in "He was despised." Hers is one of the magistral arts to be heard among today's singers. She invests every note and phrase with nobility, adding ornaments with natural ease so that they function as Handel intended. Hardly less impressive was Gene Tucker's singing both in coloratura passages and the moving pathos with which he sang "Thy rebuke hath broken His heart."

Haywood rode the familiar soprano waves with ease and good style. If she could add a feeling of tenderness to her tone in such places as "Come unto Him," she would be quite remarkable. Plishka contributed the fewest embellishments of any of the solo singers. But he made a fine, stalwart sound where it was needed, matching excellent high notes with richness in the lowest passages.

Among Shaw's virtues in the dynamics of this performance was the way he continually balanced the volume so that Russell Woollen's harpst-chord and Reilly Lewis's organ playing could he heard. For these two offered real baroque treats. Rarely has the organ part been such an outright delight as when Lewis deocrated "And He shall purify," "For unto us a Child is born," and "All we like sheep."

Momentous improvement: Before the "Hallelujah" Chorus, program asked audience to remain seated Hallelujah indeed, for the entire evening.