Mstislav Rostropovich has agreed to remain as music director of the National Symphony Orchestra for at least three more years. In a letter of intent to the orchestra's board of directors, the Russian-born conductor and cellist said he would sign a new two-year contract to take effect when his present contract expires after this coming season.

The executive committee accepted the announcement with pleasure, but again declined to release Rostropovich's salary, which remains one of the city's few well-kept secrets.

At the same time, Robert W. Alvord, chairman of the committee, announced that the orchestra will postpone its planned European tour, originally scheduled for May and June of 1979 and now pushed back to the same months of 1981, when the orchestra celebrates its 50th anniversary. Alvord said Rostropovich concurred on the delay, and that the orchestra will play concerts in Washington and other U.S. cities during the months it had planned to tour abroad.

Alvord also said, that it was Rostropovich's wish "that no discussions regarding his future with the National Symphony be held until he completed his first midwinter season as music director," Alvord said. "That season ended on May 13."