Washington Cathedral presented last night what almost certainly must have been this city's premiere of Alessandro Scarlatti's oratorio, "Agar et Ismaeli esiliati."

And if that title leaves you bewildered, rest assured that it is a familiar story. Taken from Genesis 21, it tells of the banishment of Ishmael from the house of his father, Abraham, and of the child's imminent death in the desert, when God appears before his mother, Hagar, and commands, "Arise, lift up the lad, and hold him in thine hand; for I will make him a great nation. And God opened her eyes, and she saw a well of water . . ."

"Agar" is an early work of the Neopolitan composer who became the outstanding figure of late Italian Baroque music. Last night's conductor and harpsichordist, Judith Norell, found a copy of it in the New York Public Library, and when the cathedral's choirmaster, Wayne Dirksen, called her in June about performing in the cathedral's summer festival, she grabbed the chance to do the Scarlatti. Individual parts for the players were not available, so they had to be copied from the library's edition of the complete score.

"Agar" is not a monumental oratorio like the ones Handel produced in London 50 years later. Nor does it match in scale or intensity the grand works of the mature Scarlatti, or of his son, Domenico, the famous keyboard composer.

It is more like one of those lovely but modest dramatic Baroque Bible paintings that fill the spaces in museums between massive Titians and Tintorettos. It is sophisticated in technique; it is operatic in the way the arias and duets avoid slowing down the telling of the story. There is no chorus, and it is minimal in scale -- suitable for convenient performance in the drawing room of a Neopolitan noble, which is what Scarlatti, at age 23, probably wrote it for.

Last night the 12-member orchestra was subordinate except in the grave, contrapuntal introduction. And, along with the conductor and the cello continuo, the four singers did most of the work, and there was a lot of it. The singers were short in dramatic force, considering the power of the story. Only counter-tenor Jeffrey Gall, as Hagar, maintained the illusion of his character with any consistency. The vocalizing was mostly fine, with baritone Richard Dirksen, as Abraham, particularly polished, except for some troublesome low notes. The Baroque style seemed impeccable, though how can one be sure, when the performers were probably the only persons there familiar with the music?

Next Tuesday the festival, which is free, concludes with all Bach -- ending with the "Magnificat," that joyful outpouring that usually draws a full house.