Mstislav Rostropovich announced yesterday that the National Symphony Orchestra will travel to Mexico City in October as well as to Europe, where a four-week tour is scheduled in the spring of 1979. London, Paris, Berlin, Copenhagen and Vienna are among the cities the NSO will visit.

Rostopovich, who assumed command of the symphony last year, also said that the board of directors has given permission to enlarge the orchestra by up to 10 more string players.

He was "very gateful," but pointed out that although auditions have been held, no violinist has yet been hire. "Many play," he explained at the press conference," but their playing is not good enough for future of our orchestra. So we repeat auditions in May." He said that auditions for violists had gone better, and that, "Already I think we have two new players."

The additional strings will provide, Restropovich said, "young people with very big sound -- new blood you say, to add to very good players now in orchestra."

Rostropovich spoke enthusiastically of world premieres to be given during his second season: "New flute concerto by Hovhaness, new concerto for double bassoon by Gunther Schuller. I think it very interesting to have new concertos for highest and lowest woodwind instruments."

He continued with word of new works commissioned from Witold Lutoslawski, under whose baton Rostropovich will play the Lutoslawski cello concerto; and the premiere of "Chromatokinesis" by Andrea Makris, a member of the first violin section of the orchestra. "In my opinion Marris is great composer," Rostropovich said.

The season also will bring Washington premieres of "Voices" by Antal Dorati and the Montsalvage Harp Concerto, to be played by Nicanor Zabaleta, as well as NSO performances of the Britten War Requiem, Kodaly's Peacock Variations, the Mahler Seventh Symphony, and "Oiseaux exotiques" by Olivier Messiaen.

Next season's guest conductors making first NSO appearances include Claudio Abbado, the music director of La Scala in Milan; Lorin Maazel of the Cleveland Orchestra; Leonard Slatkin, music director of the New Orleans Philharmonic; Emil Tchakarov, assistant to Herbert von Karajan, and Lutoslawski. Returning will be the orchestra's principal guest conductor, Antal Dorati; Christian Badea, the orchestra's Exxon/Arts Endowment conductor, who will appear on the regular subscription series; Leonard Bernstein, who will conduct Stravinsky's "Firebird" and "Petrouchka" in the final concert of the season; Aaron Copland, Rafael Fruhbeck de Burgos, and James Conlon.

New soloists for the season will be singers Peter Pears; Kiri Te Kanawa; pianists Peter Serkin, Murray Perahia, and Steven deGroote, the winner of last fall's Cliburn Competition; and cellist Lluis Claret, co-winner of the 1977 Rostropovich Cello Competition.

Returning soloists will include violinists Isaac Stern, Henryk Szeryng, Ruggiero Ricci, and Yehudi Menuhin; pianists Rudolf Serkin, Alicia de Larrocha and Yvonne Loriod. Soloists from the orchestra will be John Martin, cello, and Lewis Lipnick, contrabassoon. Jean-Pierre Rampal will be the soloist in the new Hovhaness concerto.

Of his career on other fronts, Rostropovich said, "I have many invitations, but I do not want to lose my cello -- I work pretty hard with my cello. So I only conduct very few orchestras outside of Washington. Next year, two weeks with the NEw York Philharmonic, one week with Boston Symphony. I have an understanding with London Philharmonic each year for five years, and small time in Paris with French National Orchestra. I do not want to do more."