Harvard-Radcliffe Orchestra (HRO) will forsake its traditional repertoire of classical music tonight as it performs a Soviet avant-garde piece never before played in the United States, HRO members said yesterday.

In an 8 p.m. concert at Sanders Theater, HRO will perform "Hour of the Soul," composed by Sofia Gubaidulina, who has won praise throughout the Soviet Union for her symphony and 50-odd ballets, scores and concertos.

Gubaidulina's music still has not been performed extensively in the United States, and HRO members have said tonight's performance will be the first of "Hour of the Soul" in thiscountry.

"As far as I know, it is the U.S. premier,"said Teresa A. Marrin '92, who has researchedGuaidulina's contemporary classical music.

Marrin, an HRO violinist, said Gubaidulina'smusic first gained a foothold in the United Statesafter she won acclaim for a performance in Bostonlast year.

"She's a big deal in Boston because she waspart of a group of [about 300] Russian composerswho came to Boston," Marrin said. "She got a lotof press even from journals in London [for theperformance] in Boston...Her work above all otherswas acclaimed.

Because Guaidulina's "extreme avant-garde"style is so different from HRO's standardclassical music, "Hour of the Soul" will challengethe orchestra, said Elliott S. Ng '91, HROpresident.

"Nonconventional instruments are scored," Ngsaid. "It requires a lot of instruments wewouldn't normally use."

"Hour of the Soul" includes a mezzo sopranovocal solo--something unusual for HROperformances--along with a percussion solo andparts for electric instruments.

"You have all these different levels," saidMarrin. "The way she combines them in this pieceis unique."

"We can't get stuck in this musical stasisplaying music that's a hundred or more years old,"said Concert Master Erich Huang '90.

HRO Musical Director James D. Yannatos chose"Hour of the Soul" after hearing the piece at theThird International Music Festival forContemporary Music in Leningrad, which he attendedas a guest of the Soviet Composers Union.

"It was the best piece of the festival,"Yannatos said. "All of a sudden she became hotstuff. The new music world knows her name.