IT IS ALL VERY WELL to take pride in Washington's much-proclaimed "explosion" in the performing arts. In at least two areas - theater and dance - Washington's variety and quality are outmatched only by New York's. But linked to this artistic profusion is an almost inevitable local problem. A great deal of what hits our stages is generated either from or for New York, or the West Coast or the European Capitals. As a result, some of our locally based institutions find themselves in regular competition with the Hureyevs and La Scalas of the world, and while a few have flourished, more have merely held their own or folded.

Against this background, the decision of the Opera Society of Washington to revamp itself as The Washington Opera (the word "Society" reeked too much of The Precious Few) and to venture upon an ambitious new course is cause for encouragement. The Opera proposes, over four years, to increase its performances almost threefold, raise the number of opera productions a year from three to at least five and improve the general level of all performances. Further, the Opera will be supplemented by 1979 by an innovative "post-conservatory" training program for young singers with "star potential." And if this plan works, another, more exhaustive, expansion would follow - putting the Opera right up there among America's increasingly numerous and upwardly mobile companies (the country's opera audience has grown about 150 per cent in the 1970s and the trend is accelerating).

Two years ago, when its directorship was accepted by George London, the Opera Society would hardly have seemed a likely candidate for such an initiative. Its financial base had never been particularly solid, its repertory had leaned too far toward the esoteric, and it had a $200,000 deficit.

Mr. London changed much of that. The deficit has been cut by almost two-thirds. The last two seasons - both sellouts - have ended up in the black. And now management feels secure in moving forward - on the theory that to stand still is to fall behind, considering this city's cultural deluge.

Mr. London has chosen to proceed gradually. Expansion of performances will be phased into four stages, with the budget gradually rising into the $2-million range. At the same time, a carefully organized and aggressive fund-raising campaign is under way, addressed to not only local sources but also the national corporations that have underwritten so many other enterprises on the Kennedy Center's stages. This money is crucial, because opera is by far the most costly of the performing arts; there is no such thing as a performance without a deficit.

It might be argued that, given these hard facts, the Opera's wisest course would be to sit tihgt and kep its head down. But since the Opera's basic financial dilemma is shared by all other American opera companies, to take that position across the board would be, in effect, to favor arresting the upward momentum of opera just as it is picking up full speed. That, we think, is too great a price ot pay - even if an end result of this boom is the eventual necessity for extensive subsidy of many financially responsible companies by government, acting as a sort of Ultimate Foundation. That, after all, is the way the bills are paid for every one of the glittering, glamorous and first-rate visiting European companies tht have given the Washington Opera such tough competition.