The National Symphony Orchestra has planned a gala kickoff for its 50th anniversary season.

On Thursday, Sept. 18, Leonard Bernstein will come out of his year-long seclusion, which he has been devoting exclusively to composing, to play four-hand piano music with music director Mstislav Rostropovich. Jean-Pierre Rampal, flying in from Paris, will play a Mozart Andante and Rondo with the orchestra, Isaac Stern will join Rostropovich in the opening movement of the Brahms Double Concerto and the Handel-Halvorsen Duo and soprano Galina Vishnevskaya, Rostropovich's wife, will sing the Villa-Lobos Bachianas Brasileiras No. 5 for soprano and eight cellos.

All this and more will be heard during a special concert in the Kennedy Center Concert Hall five days before the orchestra opens its 50th anniversary season. Also scheduled for the 18th are a White House reception, a pre-concert dinner at the Supreme Court and a post-concert supper party. Ticket prices for tier seats range from $5 to $35, with orchestra seats at $125 and box seats at $750. Admittance to one of the special events will depend on the price of the ticket. Tickets are available from the Symphony Offices, Kennedy Center, Washington, D.C. 20566.