Since the modern era of opera began here about 10 years ago, Washington Wagnerites have been left waiting, wondering when some company would do a major Wagner opera with the kind of force seen in La Scala's Verdi or Vienna's Mozart. Last night the Metropolitan Opera did it, with James Levine's magisterial, impassioned performance of "Parsifal."

Levine, the Met's music director, has been conducting "Parsifal" ever since he took over the company several years ago, and his interpretation has evolved into one that explores the range of human experience in the opera with remarkable depth.

This last work of Wagner requires a whole new set of artistic antennae both for the performer and the listener.. In "The Ring," for instance, the composer was shooting for a Michelangelesque scale. In "Parsifal" he moves to something as elusive and subjective as El Greco, complete with a Christian story. It deals with the Spanish knights of Monsalvat, whose mission it was to guard the chalice that caught the blood of Christ on the Cross and the spear driven into His body. The ignominious loss of the spear seemed to seal the knights' fate, and that is Wagner's story.

There are two common ways to approach this music. One is the utilitarian, giving an efficient overview of the psychologically complex intricacies. The other is to slowly explore all the nuances and implications of those psychological undercurrents. That is Levine's way with "Parsifal," and his total immersion in the music last night was enthralling.

It meant very slow tempos that some would not like (the opera ran for five hours and 20 minutes). It also meant a deep exploration of Wagner's themes, such as the narrowness of the line between holiness and depravity. At some moments there was thunderous sound; at others Levine doted on passages of the most un-Wagnerian delicacy.

Throughout, the Met orchestra played like the major symphony orchestra that it can be when sufficiently motivated. The chorus, too, was faultless.

Considering that two of the four leads were last-minute substitutes, the singers performed far better than could be expected.

Baritone Thomas Stewart, as the wounded knight Amfortas who must bear the blame for losing the spear, reached the happiest balance between vocal assurance and interpretive authority.

Mignon Dunn had to step in on very short notice in the soprano role of Kundry, which was to have been done by Tatiana Troyanos, who became indisposed. Last night Dunn seemed less carefully focused vocally and dramatically than she was in the "Parsifal" broadcast Saturday from New York. It was a solid characteriztion of the pathetic Kundry, doomed to what appeared to be eternal life as punishment for laughing at Christ as He was crucified.

The circumstances under which bass John Macurdy came to be singing the most grueling of the roles, the knight Gurnemanz, was even more strange. The great bass Martti Talvela had been scheduled to sing it here. But about two weeks ago he apparently suffered a heart attack while singing at the Met. His first replacement, Jerome Hines, ran into vocal problems. And the relatively inexperienced Macurdy stepped in. His voice was well up to the part's rigors, and he already showed a developing characterization.

Jess Thomas, in the title role, was once a major Parsifal, but he has fallen on vocal trouble in recent years and announced that his two Parsifals at the Kennedy Center will be his "final farewell to opera." The voice was frayed last night, but he husbanded it carefully. He was forceful, dignified and heroic.

The physical production was nothing special. It's monumental, all right, but its monumentality reeks a bit too much of science fiction. The huge flowers in the magic garden of the second act looked like man-eating plants.

The production was good enough, though, to give Levine a dramatic foundation upon which he could erect his eloquent musical edifice. The news that Levine has been asked by the Wagner family to conduct "Parsifal" at the Wagner theater at Bayreuth this summer, on the 100th anniversary of its premiere, should come as no surprise.

The performance will be repeated, at the Kennedy Center Opera House, this Friday, starting at 7 p.m.