Tchaikovsky's powerful one-act opera "Iolanta" had to wait 89 years to get its due in this country. That is what happened last night at the Kennedy Center as Mstislav Rostropovich conducted the National Symphony and some fine singers in a performance that was often spectacular. It was one of the important developments in American opera this year.

And it provided an exciting climax to the conductor's current festival of Tchaikovsky's music, with its tilt in favor of some of the finer but neglected music.

"Iolanta" has been mentioned by some scholars as the next best Tchaikovsky opera after the two well known here--"Eugene Onegin" and "The Queen of Spades." Last night's performance certainly confirmed that there is nothing farfetched about such judgments, though most of us know no more than bits and pieces from the others.

"Iolanta" shares with those two famous operas a basic Tchaikovsky theme, unfulfillable love. But the gloom and pathos of those works is only partly reflected in its vibrant music. Whereas in "Onegin" and "Queen" the tragedy gathers inexorably, in "Iolanta" we have hints from the first that somehow the seemingly unfulfillable love will be salvaged. The agent of fulfillment is love and a certain degree of fairy-tale magic that fits better with this Hans Christian Andersen fable than with the other two works by Pushkin. The story line could hardly be simpler: Iolanta, the princess, is born blind but her father, the King, forbids anyone to let her know it; a heroic Prince comes along, blunders into letting her know about her blindness but his love provides the impetus with which she gains her vision, and they live happily ever after.

Another lesson from last night's event is that there is no substitute for first-rate voices--something as true in Tchaikovsky as in Verdi, Wagner or Mozart.

It is hard to see how Rostropovich could have hired better singers for this concert version. In the title role was Galina Vishnevskaya, who sang the part at the Bolshoi during the years she reigned as its queen. Last night's performance gave perhaps the surest taste yet of the reason for her unique position in Russian music, before she and her husband, Rostropovich, had to leave. Her voice was consistently strong, and her way with the text was masterly.

There was also Nicolai Gedda as the prince, the one major tenor outside the Soviet Union who has mastered the Russian style. His voice is frayed, but he husbanded it sufficiently well that it rang out full at the climaxes. And his way with a text rivaled Vishnevskaya's.

There was also a major singing debut, that of Bulgarian bass Dimiter Petkov as the king. In his early aria, in which he despairs of his daughter's plight, the huge bass voice sounded out in considerable resonance, with evenness of range and dramatic strength. It was then that the audience broke in with applause for the first time. The aria is worthy of similar ones for low voice in "Onegin." Petkov will make his New York debut next week when Rostropovich repeats "Iolanta" there.

At least two others were very distinguished. Mezzo Lili Chookasian sang the lullaby to the unhappy Iolanta with considerable force and her usual assurance in Russian. And bass-baritone John Shirley-Quirk brought great zest and sonority to the slightly strange role of the Moorish doctor, Ibn-Hakia.

On the basis of this performance one can conclude that the pedants who have given "Iolanta" a bad name and contributed to its neglect are dead wrong. They have mused that perhaps the problem with "Iolanta" is the happy ending, that poor sad Tchaikovsky just couldn't put his heart into it. Well, any fool who bothers to know "The Sleeping Beauty" knows that's nonsense. It seemed to me that it's at the point in the opera where things begin to look up for the blind princess that the music really catches fire and blazes on to the end. Only in the final "Hymn to Light" did the composer seem to fall a little short; he sounds like he's under the influence of a Verdi triumphal scene, but doesn't quite match it.

The reduced orchestra played stylishly and, after the first few minutes, did not drown the singers, who were assembled on risers at the back of the stage. Rostropovich's grand performance will be repeated here tomorrow night and Friday.