Leonard Bernstein will conduct the National Symphony Orchestra on its Fourth of July concert at the Capitol grounds this year -- a spot that has been traditionally reserved for Mstislav Rostropovich. The announcement, made yesterday in a NSO news conference, said that the program will include Bernstein's "An American Songfest," which the orchestra has recorded.

Rostropovich said that when he learned Bernstein had expressed an interest in the concert at the Capitol, he immediately agreed because "I think it is appropriate for America's greatest musician to conduct the National Symphony there on the nation's birthday."

Other highlights of the NSO's 1985-86 season -- including a four week-tour of Europe next September, guest conductors and soloists and three premiere performances -- were also announced.

The orchestra will give the American premiere of Krzysztof Penderecki's recently completed "Polish Requiem," parts of which Rostropovich conducted in the 1983-84 season. There will also be the world premiere of the Fifth Symphony of Finnish composer Aulis Sallinen, which was commissioned by the orchestra, as well as the first performance of "Many Moons," a concert overture by American composer Dan Tucker.

In the summer, there will be three al fresco concerts on the West Front of the Capitol, including the one on the Fourth. The annual Wolf Trap series will include a four-concert Beethoven festival, two pops concerts and the annual all-Tchaikovsky celebration.

Rostropovich, the NSO's music director, will conduct here for 10 weeks and play the cello during a week devoted to the works of French composer Henri Dutilleux. Principal guest conductor Rafael Fru hbeck de Burgos will direct for five weeks. Michael Tilson Thomas, Yoel Levi and Jean-Pierre Rampal will each direct for two weeks.

At the Kennedy Center news conference, orchestra officials also said Andrew Litton, who is 25 and has been the orchestra's Exxon/Arts Endowment conductor since 1983 on an arrangement jointly subsidized by those two sponsors, will take the position of associate conductor, replacing Hugh Wolff.

Wolff, 31, preceded Litton as the NSO's Exxon/Arts conductor. He will continue conducting the orchestra, as well as guest-conducting in this country and in Europe.

Rostropovich described Wolff as "one of the greatest talents of his generation in the United States." Rostropovich added, "We are proud that we opened the door for him. I anticipate battles between many orchestras that want to take him for their conductor. I really think he is ready."

Wolff will continue to serve as music director of the Northeastern Pennsylvania Philharmonic. Among his upcoming engagements: the Chicago Symphony, the Los Angeles Chamber Orchestra and the Stockholm Philharmonic.

The NSO's two-week tour of American and Canadian cities begins March 18. The second of the orchestra's "Tour America" series, it starts in Toronto and ends in Philadelphia.

The European tour will be the second under Rostropovich (the first was three years ago). It will be timed to visit some of the summer festivals, including those in Lucerne, Montreux, Berlin, Athens and Flanders. The orchestra will visit Yugoslavia for the first time, playing in Belgrade and Ljubljana. Other cities planned for visits include Paris, Frankfurt, Hamburg, Du sseldorf, Stuttgart and London, where the concerts will be subsidized by Sultan Qaboos bin Said of Oman. The sultan is dedicating his support of those concerts to First Lady Nancy Reagan, in whose name he had previously endowed the NSO with a chair of narrative music. The tour is being underwritten in part by Mars Inc.

Former music director Antal Dorati also will return to the orchestra he led for seven seasons. Other guest conductors include Klaus Tennstedt, Leopold Hager, Serge Baudo and Peter Maag.

Other debuts with the orchestra: pianists Jeffrey Kahane, Jean-Ives Thibaudet and Shura Cherkassky; singers Elizabeth Knighton, Martha Senn, Jerry Hadley, Carolyne James, Kevin Langan, Frank Kelley and Wieslaw Ochman, and violist Marcus Thompson, winner of the 1980 National Black Music Colloquium and Competition at the Kennedy Center.

It was also announced that the opening week's concerts would be financed with a contribution from Raleighs.