Rafael Fruhbeck de Burgos gave his first concert as principal guest conductor of the National Symphony Orchestra on Saturday. The thrilling, popular program offered both the genuinefolk rhythms of Dvorak's Slavonic Dances and the exotic stylization of the original folk impulse created by Carl Oriff in his Carmina Burana. The maestro also introduced the tens of thousands gathered at the West Lawn of the Capitol to one of the finest choral ensembles ever to sing with the NSO.

Oriff's Carmina Burana is not a piece that invites profound musical or intellectual analysis, but it does encourage listening. In it the composer posed in the garb of simple rhythms and simpler melodies, and succeeded in capturing the essence of the bourgeois sensibilty at its best.

The Orfeon Donostiarra of San Sebastian, Spain, is one of those groups that can turn Carmina Burana into a great musical experience. Its bass and tenor sections rank with those of the greatst operatic choruses heard here in past seasons, while its female voices-particularly the sopranos-are blessed with that purely Spanish tiple sound, which penetrates sweetly through any orchestral texture without once becoming reedy or metallic. The Orfeon's diction was impeccable, even through thethe abominable outdoor amlplification. They must be heard here again, with the generous acoustics of the concert hall on their side.

Soprano Ruth Welting and baritone Brent Ellis were an extra luxury for the evening. Weltin's voice has matured into a gift of silver and silk, and more than met the stratospheric demands of the "Dulcissima" section with effortless vocal resources and almost insolent technique. Brent Ellis displayed his warm baritone with ease and elegance, and with a new booming evenness at the top.

The orchestra responded to Fruhbeck de Burgos with excitement and vigor. His appointment to the NSO is good news indeed.