ANTAL DORATI'S BOLD, bracing stewardship of the National Symphony Orchestra comes to an end with this week's four concerts, concluding an era that was by far the orchestra's richest since its founding. It is thus a time to note that the orchestra and audiences alike owe a very great deal to this eminent man who seven years ago - when approaching the peak of his musical powers and of his career possibilities - took on the potentially thankless task of breathing new life into a Washington ensemble that was listless and atrophied. Coming here then was something Mr. Dorati clearly didn't have to do. He was solidly based with the Stockholm Philharmonic - the latest of a succession of American and European music directorships to have come his way. (Next year he will add to this list the Detroit Symphony.) He also composed in the summers. And he derived considerable income from recordings that poured out at a prodigious rate. (His most distinguished undertaking may well turn out to be the monumental complete recording of Haydn's 100-odd symphonies - most of them not otherwise played, much less recorded, by major conductors.)

As our critic Paul Hume once noted, "The National Symphony needed him far more than he needed it." The adventurous Mr. Dorati, though, found himself lured by the challenge. He plunged in, headstrong and innovative, as if it were primarily his reputation, and not the orchestra's, that was at stake. If the orchestra he inherited could then have been regarded as big league at all, its ranking was firmly in the cellar. But Mr. Dorati quickly steered the musicians on a rigorous course that has put them within, shall we say, striking distance of the top.

It is a simple, impressive fact that someone now hearing the National Symphony for the first time in years would have trouble recognizing the sound as that of the same orchestra. And the same may be said for the symphony's repertory, which, Mr. Dorati discovered on his arrival, was astonishingly meagre. And so he had a programming field day: In 75 programs spread over seven years, he conducted 64 works new to the orchestra; there were 22 world premieres and four additional American premieres. One season he scheduled a work new to the orchestra on each week's program. During 1976 an American work was played weekly. For us and for Mr. Dorati, it was quite an adventure.

Mr. Dorati, who will return next season for six weeks as principal guest conductor, insists that during the past seven years he was merely "a conduit for the players." Perhaps he would prefer simply that this week's performances speak for themselves; the orchestra is playing the vast, majestic Mahler 5th. "I wanted something joyous," he said recently, "that would permit the orchestra to show itself at its most expansive best." That "best" is Mr. Dorati's matchless legacy here; he is right to end on a joyous note.