Rafael Fruhbeck de Burgos will become the principal guest conductor of the National Symphony Orchestra next season, music director Mstislav Rostropovich announced at a press conference yesterday.

Taking a position recently held by Antal Dorati, Fruhbeck will conduct for at least six weeks during each of the next three seasons. In recent years, he has been one of the most frequent and one of the most widely acclaimed guest conductors at the NSO.

The 1980-1981 season, which will mark the orchestra's 50th anniversary, will begin with a gala benefit concert on Sept. 18. The programming will have more variety and more emphasis on American music than has been seen since the Bicentennial season. There will be world premieres of new works by Peter Mennin, Alberto Ginastera and Andreas Makris as well as the American premiere of Krzysztof Penderecki's "Te Deum" and the Washington premiere of Leonard Bernstein's Symphony No 3 ("Kaddish"), which the composer is revising.

Both of the NSO's living former music directors, Dorati and Howard Mitchell, will conduct programs during the 50th anniversary season, and the orchestra's founder, Hans Kindler will be commemorated in the first program of the year. That program will include Kindler's transcription of a Frescobaldi toccata and Piston's symphony No. 2, as well as "Stars" by Mary Howe, a Washington composer who played a major role in getting the orchestra started.

Other guest conductors will include Max Rudolf, Kiril Kondrashin, Leonard Bernstein, Eugene Ormandy and Erich Leinsdorf. Hugh Wolff, a young conductor sponsored by grants from Exxon and the National Endowment for the Arts, will conduct Peter von Winter's concertino in E-flat, with Rostropovich and chief clarinetist Loren Kitt as soloists, during the week of Nov. 18. Guest soloists making their first appearances with the orchestra will include pianists Murray Perahia, Maurizio Pollini and Youri Egorov, violinist Silva Marcovici and guitarist Narcisco Yepes.

Besides the Sept. 18 benefit concert, there will be two special non-subscription concerts during the season: a celebration of Aaron Copland's 80th birthday on Nov. 14 and a performance with pianist Rudolf Serkin on Feb. 13, 1981.

Rostropovich will spend 12 weeks with the orchestra during the coming season: 10 exclusively as a conductor, one as a cello soloist and one as both conductor and soloist. Under the baton of Max Rudolf, he will join violinist Itzhak Perlman in the Brahms Double Concerto during the week of Nov. 11.

The season's emphasis on American music will include the NSO premiere of Chadwick's Symphony No. 2 on a program with works of Sowerby, Griffes and MacDowell, the week of Feb. 10, 1981. A week earlier, Eugene Ormandy will conduct the Symphony No. 3 of Roy Harris. The centennial years of two composers who emigrated to the U.S. will be observed during the season: Ernest Bloch in 1980 and Bela Bartok in 1981.

Answering "rumors that say I do not stay here long enough working with orchestra," Rostropovich said that during the 1981-82 season, he plans to spend 19 weeks with the NSO. "Then," he said, "there will be maybe two conductors in the United States who spend more time with their orchestras than I do."