BELA BARTOK'S Concerto for Orchestra, one of the great composer's last works and one of his most immediately enjoyable, is being conducted in Washington by the first musician who saw the finished manuscript: Antal Dorati, this week's guest conductor with the National Symphony.

Dorati, a friend of Bartok's, was summoned to Bartok's apartment in New York when it was finished. Dorati looked over the score, concluded that it was "an exceptional masterpiece" and arranged to have the American Ballet Theatre buy the ballet performing rights -- supplying some badly needed funds for the impoverished, exiled composer.

Bartok was ill, running a constant temperature and seriously underweight in 1943 when he wrote the Concerto for Orchestra. But he finished the work in less than two months, and there are few signs of his unhappy condition in the music itself, which bursts with energy in the last movement and balances its moments of deep seriousness and poignant nostalgia with sardonic wit and instrumental brilliance.

Dorati has recorded it several times, always with great impact and most effectively of all in his latest version. This recording (Philips 411 132-1) has just been awarded the Edison Prize by a jury of music critics, and the award is richly deserved. The recording orchestra is the Concertgebouw of Amsterdam, a virtuoso ensemble suitable for this virtuoso music; the rhythms are sharply profiled, the accents vigorous, the tone rich and well-balanced.

CONCERTO FOR ORCHESTRA -- Dorati conducting the Concertgebouw Orchestra, with Bartok's "Two Pictures" Op. 10 (Philips 411 132-1); Dorati conducts the NSO at the Kennedy Center on Friday, Saturday and Tuesday. Also on the program: music of Haydn and Brahms.