THE OPERAS OF Peter Ilyich Tchaikovsky have been slow to emerge from their Russian isolation and become known to listeners in the West. Last night's performance of Tchaikovsky's "Iolanta" conducted here by Mstislav Rostropovich was not a premiere. But for most persons at the Kennedy Center Concert Hall it might as well be.

So far as conducting is concerned, it is even new to Rostropovich, despite his enormous authority in Russian music. At least one member of Rostropovich's cast has sung it before, soprano Galina Vishnevskaya, who sang the title role when she was the leading star of the Bolshoi Opera. There is a recording of "Iolanta" with a Bolshoi cast. By the time it was made, neither Vishnevskaya nor her husband, Rostropovich, was in favor with the commissars. And at a party several days ago, Vishnevskaya declared the recording to be "not good." So much for that.

One way Rostropovich has improved upon that version has been to restore all references to the deity, which were removed by his foes, the commissars.

Also, it should be stipulated that despite the apparent surface similarity of their titles, "Iolanta" has absolutely nothing else in common with Gilbert and Sullivan's "Iolanthe" except a name.

Tchaikovsky's opera, which is in one act and lasts about 90 minutes, has its origins in a tale by Hans Christian Andersen, about a princess who is born blind but not permitted to know it by her father, the king of Provence, and who eventually gains her vision through the love of a hero, Count Vaude'mont, a Burgundian knight.

"Iolanta" opens with a somber prelude. Then, with that kind of dramatic contrast so typical of Tchaikovsky in his ballets, it rises on a brilliant garden, outside the palace of the King of Provence. The princess is picking fruit with her retinue. She finds the fruit by touch and puts it in her basket, but gradually she retreats into herself, with an aria about the unexplained sadness that has overtaken her. Her nurse and her friends lull her to sleep in a comforting trio.

Retainers appear to announce the king's arrival. The king tells a Moorish doctor, Ebn-Hakia, that he is Iolanta's last hope, and the doctor goes to look at her as she sleeps. Then the king sings an aria in which he pours out his grief about his daughter's affliction.

Ebn-Hakia returns to announce that to be cured the princess must be told of her blindness and must develop the will to be able to see. The king remains unconvinced.

As night falls two men who have lost their way enter, Robert, Duke of Burgundy, and Count Vaude'mont. They do not realize where they are. Robert bemoans the prospect of a loveless marriage to his betrothed, Iolanta, whom he has never seen, and he hopes the king will release him from his vow, so that he can marry his new beloved, Mathilde.

They come upon the sleeping Iolanta. Vande'mont is carried away by her innocent beauty. Robert warns of a trap and leaves to fetch their companions. Iolanta wakes and offers Vande'mont a glass of wine.

The duet that follows is the musical and dramatic centerpiece of the opera. Vande'mont asks Iolanta for a red rose to add to the white one that she has already picked for him. She brings him another white one. Suddenly Vande'mont begins to understand that she is blind. "Is there no desire in your heart to see the light and glory of the universe?" he asks. "What does the word 'see' mean?" she responds. In a surge of ecstasy, he describes the beauties of light.

The king and Ebn-Hakia enter, astonished to find her there with a young man, and one, she tells them, who has given her an idea of the meaning of the word "light". The king is horrified, but the Moorish doctor sees the possibility that this could be the cure. The king begins to understand, and says Vande'mont will die unless Iolanta is cured. Iolanta proclaims her determination to see and to save the life of the knight, with whom she has fallen in love.

Robert returns to admit his secret. Then Iolanta begins to see. The king relents, releasing Robert from his vow and gives Iolanta's hand to Vande'mont. The opera ends with a hymn to light.