Mstislav Rostropovich will open the National Symphony's Orchestra's 49th season on Tuesday, Sept. 18, the earliest opening in the orchestra's history. Before the season is over, the orchestra will have made some milestone additions to its history, both musically and geographically. Let's take geography first.

The orchestra's subscription dates at the Kennedy Center have a gap between the concerts of March 28 and April 29. During those weeks, the National Symphony is planning to play in Japan and possibly China. While the orchestra has been to Europe twice in the past, if briefly, once to South and Central America, and just last spring to Mexico, this is its first trip to Asia and its first major international tour under Rostropovich's baton.

On the musical side of the record, the orchestra's upcoming season is highly conventional. Its highlights will include its first performance of the Mahler Eighth Symphony in the Kennedy Center, an event of grant proportion, scheduled for April 29 and May 1 under Erich Leinsdorf's direction.

Antal Dorati had considered at length the desirability of performing the big Mahler -- it is sometimes called the "Symphony of a Thousand" -- for the opening of the Kennedy Center Concert Hall in 1971 but eventually decided against it because, in order to accommodate the required large double chorus, eight soloists and enlarged orchestra, it is necessary to build an extension to the state. Dorati felt, quite properly, that the general public's first view of the new Concert Hall should not be during a temporary alteration.

Paul Callaway has conducted the Mahler Eighth twice in Washington Cathedral, where the vast spaces heightened the effect of certain aspects of the work while obscuring some details. Julius Rudel gave the symphony at Wolf Trap several summers ago, but the immense impact of Mahler's epic writing was seriously dissipated in the outdoor air.

While the National Symphony has not yet announced the details of the Mahler logistics -- the choruses and essential soloists -- it is exciting any time this monumental work, introduced to this country by Leopold Stokowski a half century ago, is in prospect.

It is a vital testimonial to Leinsdorf's popularity with both the players of the orchestra and its public that, having just led the orchestra on the Capitol lawn, he will return again in October for three concerts and once more for the Mahler in April.

The year's other guest conductors include Rafael Fruhbeck de Burgos, another musician of outstanding persuasiveness with musicians and listeners; young Kazuyoski Akiyama, Christian Badea, Robert Shaw, Kiril Kondrashin, Max Rudolf; and, in his final season as the orchestra's principal guest conductor, Antal Dorati.

Dorati, despite his duties in Detroit and London, will conduct here for five weeks, bringing with him Beethoven, Haydn, Strauss (Heildenleben,) Barber (the Piano Concerto,) The Bruckner Seventh and a great choral evening with the Kodaly "Psalmus Hungaricus" and Walton's "Belshazzar's Feast."

For his own part in the orchestra's programs, Rostropovich will be in charge of nine weeks of concerts. This is less time than the music director of an orchestra should spend in a season of 24 subscription concerts. Surely Rostropovich and the symphony board should move toward the time when he would be in charge of not less than half the season, and preferably more. The world's greatest orchestras have always been, and still are, those who play most of the time for the fewest conductors.

There is another serious disproportion in the National Symphony programs this year: In all 24 programs, six works by American composers are announced, while 14 by Russian composers are scheduled, two French, 26 from the Austrian-German areas, two English, two Spanish, two Polish, two Italian, two Scandinavian (one each from Sibelius and Nielsen,) one Czech and one Hungarian.

The principal shortcoming, as usual, is in the realm of American music. The National Symphony is failing in its obligation to our music of yesterday and today and to its position in the nation's capital by offering its subscribers only the familiar names of Schuman and Barber, and the less wellknown Persichetti, Druckman and Makris.

With the orchestra's 50th anniversary season beginning next year, there still remain a shocking number of American works which other orchestras have played but the National Symphony has not: Pulitzer-Prize winning compositions, symphonies, tone poems and concertos forming a magnificent literature that continues to be denied National Symphony audiences. The symphonies of Walter Piston, Howard Hanson and Roy Harris, of Leo Sowerby and Douglas Moore, Randall Thompson, of Peter Mennin and Stephen Burton, are as worthy of place on NSO evenings as those of Tchaikovsky, Shostakovich and Prokofiev. So, too, are tone poems of Henry Cowell and Michael Colgrass, Charles Griffes, Charles Ruggles and Ernest Bloch. And what of the beautiful unheard music of Charles Tomlinson Griffes?

John LaMontaine's Piano Concerto won the Pulitzer Prize 20 years ago when Jorge Bolet played it with the National Symphony. Where has it been since then? There are concertos by Martin Mailman and Quincy Porter never heard here, and the neglected Violin Concerto of Samuel Barber, the Piano Concerto by Amy Marcy Beach and Ernst Bacon's historic poem, more important to this city than any other, called "Ford's Theater."

It's a strange thing, but some conductors, if asked why they do not play either the great Fifth or Sixth Symphony by Walter Piston, will reply: "I've played it." They never think of saying that about the Seventh Symphony of Beethoven, even after playing it for the 100th time. Rostropovich is in a unique position to give Washington a historically memorable season next year. For the National Symphony's 50th anniversary, nothing else will do.