PITTSBURGH (AP) _ Rock 'n' roll singer Frank Zappa has pledged to register fans at his concert here tonight to vote, and the League of Women Voters couldn't be happier.

The alliance prompted one elderly league member to joke that he would turn down the volume on his hearing aid during the concert, said Pittsburgh League President Marsha Bingler.

''I consider that an upbeat comment,'' said Ms. Bingler. ''The gentleman who said that is about 70 years old. He does have trouble with his hearing.

''I've had no one in the league say anything other than that this is a worthwhile effort,'' she said. ''The league encourages the widest participation in the electoral process.''

Zappa said 400 people registered at his recent concert in Boston, and about 380 registered at a stop in Hartford, Conn.

''The only way to change what is going on is to vote,'' he said. ''Unless young people get involved, their decisions will be made by people older than them who don't know or don't care.''