THE NATIONAL SYMPHONY has ended its

fiscal year in the black--a hopeful sign, as well as highly unusual. Throughout most of its long life, the orchestra has teetered along the brink of financial collapse. The quality of the performances has risen to a high level over the past several years, but quality in music, as in most other things, does not come cheap. Resources lagged behind. A year ago the orchestra was still running its customary large deficit.

Fund-raising has been, in the past, peculiarly difficult in Washington. Average incomes in this city are high, but because it is not an industrial city Washington has never produced the great blocks of wealth that endow universities, museums and orchestras. There have been notable exceptions, but generally speaking the traditional methods of recruiting support for the arts have worked badly here. The orchestra's recent success reflects interesting changes not only in its fund-raising strategy, but in the economy of Washington as well.

Over the past decade there's been a rapid increase in the number of business corporations with substantial operations here. The orchestra has been in touch with a good many of them with the help of the Third Sector Project, a nonprofit organization the purpose of which is to help other nonprofit organizations in the struggle to balance their books. Corporations, and accounting and management firms, first donated analysis and advice in the areas in which they are skilled--marketing, organization and financial planning.

Ticket sales rose. Private contributions from ticket-holders rose. Then corporate contributions rose. That's the sequence in which it usually goes, since corporations' donations rarely reflect one person's taste but rather require a showing of public support. That, incidentally, is also why individual gifts are important even when they aren't large.

You can't say, unfortunately, that the National Symphony's financial troubles are over. The past year's income included a federal contribution of $1 million--and it isn't likely to be repeated. The orchestra's budget, like all budgets, rises with inflation, and ticket sales cover less than half of it. But the music is reaching wider audiences, and it's getting unprecedented support from the business corporations that are a growing presence here. This city has not yet fully answered the question whether it is prepared to support an orchestra that is now first class. But the current progress is promising.