In the 19 years of its existence the Tapiola Choir, which sang at Washington Cathedral last night, has become a sort of Vienna Boys (and girls) Choir of the North. The Finns have sent the 50-plus young members abroad on 23 tours, and Deutsche Grammophon has found the group's recordings to be great hits. Last night's performance was part of the Tapiola Choir's third American tour--as a Finnish contribution to the "Scandinavia Today" arts festival that will be going on throughout the country in the next months.

Given the public relations value that Finland places in this charming entourage, it was a surprise that last night's most arresting work was by a Swede, the contemporary composer Arne Mellna s. His short little tone poem of a song, "Aglepta," was delightful proof that the far-out aleatory and deliberately theatrical devices he learned from his teacher, Gyorgy Ligeti, can be just as charming as "Peter and the Wolf" when used with wit.

The text is a "magic spell" from 19th-century Sweden. "Say these words to your enemy: 'Aglaria pidhol garia Ananus Quepta,' and blow at him so that he does not know where to go or what to say." Instead of making it esoteric or arcane, Mellna s sets it literally. When the time comes to blow at the enemy, that's exactly what the choir did. And when the enemy was portrayed, the choir made high-pitched squeals, like an approaching flock of birds. All this was just fun, from a composer who first came to the attention of many several years ago when he included bursting balloons in one of his works.

Another interesting work was an unaccompanied set of four "Songs from the Sea" by the contemporary Finnish composer Aulis Sallinen. The first and last of these were folkish settings of actual folk verses. The third, "I Don't Praise the Sea," also had folk origins, but the music had some of the stark ascending and descending octaves and other intervals that one has come to think of as Finnish, because they are so essential to the works of that giant of Finnish music, the late Jan Sibelius. One listens for this kind of thing in any Finnish work, because most people's knowledge of Finnish music begins and ends with the works of Sibelius. Even the choir itself is named after the Forest God of Finnish mythology, who was the subject of Sibelius' last major work.

The program closed with that most famous of Finnish works, the hymn based on Sibelius' final hymn in "Finlandia," which has become a second national anthem in that country.

Unfortunately, further exploration of the Scandanavian music promised in the written programs was impossible. The works were not performed because the concert was foreshortened in order to get the large crowd out on the slopes of Mount St. Alban's in time to watch the 9 p.m. fireworks on the Mall. Thus, settings of Lorca poems by Einojuhani Rautavaara and a short song by Harri Wessman were left unperformed.

For diversity there was some Bach: the "Domine Deus" from the B minor Mass and a movement from the A minor unaccompanied violin sonata played by the agile young Pyry Mikkola. And there were also three American spirituals.

The choir makes a quiet, lovely lyric sound. In the big moments, like "Finlandia," it lacked the necessary power, but in something like the enchanting Mellna s song it could hardly be bettered.