IT WAS IN the summer of 1943 in wartime Washington that David Finley, then the director of the National Gallery, said to Richard Bales' wife, Betty, "The music is just getting to be too much! You tell Dick I want him to take complete charge of all of it!" A little simple arithmetic will show you that Bales is now ending his 36th year "in charge of all of it."

When Richards Bales gives the cue for the beginning of his Music of the American Revolution at 7 p.m. next Sunday in the East Garden Court of the National Gallery of Art, he will be sounding the opening of the 36th annual festival of American music to be given there under his directorship.

The concert will be the 1,541st in the Gallery's history. Most of them have been presented under Bales' general supervision. "I was not here for the first concerts," he recalled recently. "They were being presented very alfresco, under the direction of George Gaul." (Gaul was a violinist in the National Symphony, for many years that orchestra's personnel manager, and also in charge of the pit orchestra at the National Theater.)

"I have not been responsible for all 1,541 of the concerts," Bales said, "but I tried to check on the precise number that were given before I came in.

"I had been a student at the Eastman School where Howard Hanson had regularly presented festivals of American music, and I made up my mind that if I ever got into the right position, I would do the same thing." When the National Gallery opened up for Bales as just "the right position," he started the American festivals in his first spring, that of 1944. They have continued without interruption since.

"Over the years, the audiences at the American festivals have gotten bigger and better," Bales said. "And I have noticed that the more way-out pieces have gone over more successfully. We have not made a habit of doing the extreme avant-grade, but our concerts are broadcast over station WGMS, and our American programs have not brought in complaints."

With a reflective look in his eye, Bales talked about the concerts during World War II. "There were of course lots of soldiers and sailors and Marines and WACs in our audiences. Now it has gotten to the place where we get letters saying that this couple met or got engaged at one of the concerts."

It was the success of the daytime concerts in London's National Gallery that suggested the idea for something like them at the Gallery in Washington to director Finley. The concerts have always been free. Over the years, they have been paid for out of the income of various bequests to the Gallery. The first of these was the Andrew W. Mellon Fund. In later years, similar gifts from the Calouste Gulbenkian and the William Nelson Cromwell Foundations have helped to foot the bill.

"For some years now," Bales noted, "the American Festival concerts have been paid for out of the F. Lammot Belin Fund." There will be eight of them this year, opening and closing with concerts by the National Gallery Orchestra, which Bales has conducted since he took over the choice position. "There is still one member of the orchestra who has played in it all those years," he said. "He is the second clarinet, Herman Olefsky, and he looks almost the same today as he did back then."

As he has always made a point of doing, Bales sets up a variety of kinds of concerts for the American festivals. This year there will be, in addition to the orchestral programs, concerts by the American Chamber Trio, by pianist Claudia Stevens, violinist Bruce Berg, the Audubon String Quartet, by cellist Harry Clark, and an electronic concert presented by husband and wife Barton and Priscilla McLean from the University of Texas at Austin.

Putting his philosophy about music at the National Gallery into regular practice, Bales has followed the rule of "letting them do the program that shows them off to their best advantage." He added, "We can't do everything, but we do the best we can."

That "best" has brought the music of almost every prominent American composer of this century to the Gallery, as well as that of many lesser known figures. Through the devices of inviting singers, various chamber ensembles, and choruses, and soloists to the American Festival, Bales makes it possible for audiences to hear the widest possible range of this country's musical thought.

In the course of the dozen or so orchestral programs that he conducts each year, Bales has led such notable world premieres as that of Charles Ives' First Symphony and the Washington premiere of the Ives Second and Third Symphonies.

World-renowned soloists have played at the Gallery in the past 3 1/2 decades. It was there that French pianist Philippe Entremont made his United States debut. And it was there that great American pianist John Kirkpatrick first brought Washington audiences his unmatched performance of Ives' Concord Sonata.

But Bales has continually offered National Gallery music lovers a wide range of the world's music reaching back to Purcell, Telemann and Handel, and up to recent works of Europe's best-known composers. His performance of the "O", or "Nullte," Symphony by Anton Bruckner in the Nowak Edition, later followed by that composer's First, Second and Third Symphonies, was an American first.

"Over the years," Bales summed it up, with justifiable pride, "we have done literally several thousand world premieres and Washington premieres." He had often highlighted the music of Washington composers, both from the past generation of R. Deane Shure, LaSalle Spier, Bainbridge Crist and Mary Howe-"She wrote what I think is her finest orchestral piece, 'An Agreeable Overture' for us," Bales recalled-and more recent prominent Washington composers Emerson Meyers, Russell Woollen, Robert Evett and Robert Parris.

As a composer and arranger, Bales has made significant contributions both to the Gallery concerts and the recording world. His three patriotic cantatas, "The American Revolution," "The Confederacy" and "The Union," were all recorded by Columbia.

In tribute to the magnificent surroundings in which he works, he has composed several National Gallery Suites, inspired by paintings that hang near him as he composes.

Like every planner of programs, Bales always has an eye out for the future: "I spend an awful lot of time going over scores." The top of the Steinway in his office was covered with many, but he said, "The load on the piano today is a light one. . . I always want to make each program a good one on its own-not just get up a program that has lots of asterisks meaning 'first perormance.' And I always choose things I know the orchestra will do well and show off well.

"I have tried to go around the edges of the repertoire, doing things that are not done all the time. You know how the standard repertoire is overworked."

Asked if there were still things he would like to do that he has not yet done, Bales, who studied conducting with Serge Koussevitzky, had a ready answer: "Yes, as you know the East Garden Court is not suitable for stage works. I have always thought I would like to do some works for the stage."

What about the stage in the auditorium of the new East Building of the Gallery?

"The decision had been made thus far," Bales began, "to keep our concerts here in the East Garden Court. Any concerts in the new wing would be directly connected with some specific event."

At this point, however, a kind of speculative gleam crept into his eye, and he went on, "That might be a good place for a stage work." Hearing Bales refer to the "three princes" for whom he has worked during his years in charge of music at the National Gallery-David Finley, John Walker and now Carter Brown-I wondered if, within his suzerainty, he might not have precisely the power he seeks, "to do a stage work." It seems a fine idea.