AS THE National Symphony Orchestra comes to the end of its 50th anniversary season, it has never stood taller artistically. One severe cost of this success, though, is that its debts are now standing equally tall.

Orchestras are, almost by definition, not profit-makers. Like every other major orchestra and opera company, the National Symphony loses money every time it performs, no matter how many seats it sells.

This season it costs about $8 million to run the NSO. And, just as the budget is up, so is the projected deficit -- $1 million. Last season's was $750,000.

To balance the books, the symphony's board of directors entered last week on a long-planned campaign to raise sufficient funds to provide a permanent solution to the orchestra's financial problems. The board is seeking an intial sum of $3.25 million, both to pay off the deficits and to meet the emergency challenge grant of $1 million made last December by Congress, a grant to be matched at the rate of $2.25 for each federal dollar. $750,000 has already been raised, and the deadline on matching the challenge grant is next Sept. 30.

Beyond that figure the board is undertaking a drive to raise between $5 million and $11 million to provide long-term stability for the orchestra. Such a figure is small compared to the endowments of such orchestras as Pittsburgh's and the Minnesota, whose reserves are between $30 million and $40 million.

Unlike such other cities, however, Washington has not, and never has had, the regular support of large corporations whose bases are the home cities of other major orchestras. Thanks, however, to persistent work on the part of recent boards of directors of the NSO, some large corporations, becoming increasingly aware of the significant role played by the National Symphony as the orchestra of the nation's capital, have made substantial contributions to its support, both in cash and in valuable professional services. NSO president Leonard Silverstein said recently that, at a time when there is a question about how much federal support may be available to the arts in the immediate future, he is going to press for more corporate and individual support than ever.

Ironically, as debts rise so does revenue from income-producing activities -- ticket sales and subscriptions. The orchestra sold 16,053 series tickets compared to 13,821 the previous year, with a value of $1,260,081, over $1,202,066. Its total seat sales numbered 153,396, compared to 138,838 the year before. In the critical area of single-seat sales, everything was similarly up: In the first 19 weeks of the season now ending, which is all but the final week, 40,765 single tickets were sold, against 37,717 last season, producing $460,417 compared to $350,857.

For several concerts this season, the price for individual tickets was raised, which brought in $17,858 from the Perlman-Rostropovich and Bernstein concerts, and which will, at a conservative estimate, bring in an added $25,000 from the final week of Beethoven Ninths.

In pursuit of larger audiences next season, the orchestra has made major changes in its performance schedules. In place of the present Tuesday-Wednesday-Thursday evening concerts, subscription series will be played on Tuesday-Thursday-Friday nights. The eight afternoon matinees, long played on Fridays will move to Wednesdays, and a new series of five Sunday matinees is planned. The Thursday evening programs will begin at 7 rather than 8:30.

Artistically, the National Symphony has now reached a point where, when it is "up," it can stand beside all but three for four of the world's greatest orchestras.

And with the completion of Rostropovich's fourth season as NSO music director, NSO artistic goals for the future are bright. An essential European tour, long-delayed, is now scheduled for February 1982, during which the orchestra will play in many major European capitals. The heads of state of each of these countries have announced they will attend.

Rostropovich takes justifiable pride in pointing out that while the National Symphony, like other leading American orchestras, commissions new works from American composers -- Mennin and Druckman being recent examples, while Leonard Bernstein who does not accept commissions, has brought several of his works to the National Symphony for their world premieres -- the NSO also has an eviable history of recent commissions to major European composers: Ginastera, Lutoslawski, Linduski, and Walton, in addition to being the first U.S. orchestra to perform the "Te Deum" of Krzysztof Penderecki.

With its artistic position well established, nothing could more fittingly reward the National Symphony Orchestra's reaching the age of 50 than to see its financial position equally solidified. The musicians, the board and the audiences deserve that assurance.