Michael Bloomfield, 37, the electric guitarist and featured performer of the Paul Butterfield Blues Band and a leader in the revival of urban blues music, was found dead Sunday in the passenger seat of his car in San Francisco.

Authorities said a bottled labeled "Valium" was found on the seat next to Mr. Bloomfield. They said the death appeared to be accidental and an autopsy was scheduled.

Mr. Bloomfield played lead guitar with the Chicago-based Butterfield band starting in the mid-1960s. He later appeared with other groups, including Electric Flag and KGB.

Mr. Bloomfield and other white musicians took the urban blues style honed by such black musicians as McKinley (Muddy Waters) Morganfield and blended it with rock music.

The hybrid music focused new attention on the blues and resulted in the professional resurrection of a number of black blues musicians.

Mr. Bloomfield was the son of wealthy Chicago parents. His early years of success included appearances at the legendary Woodstock and the Monterey pop festivals. His career flagged during the mid-1970s amid tax difficulties, professional inertia and drugs.

In 1974, he took a job composing and performing music for the soundtracks of pornographic movies. His most recent concert performances were mostly nostalgic get-togethers with musicians he had worked with in earlier and happier days.

"Maybe there a few moments of ecstacy," he once said of his life as a musician, "but the price you pay for that is pure daily hell. And it's true the moments of my greatest creativity came out of intense agony -- when I'd been on the road for months, strung out, junk sick."

He said, "I know it's not worth it. I have to find a balance in comfort and deal with art on a daily, human basis."