Gary Duncan, a guitarist, singer and songwriter best known for his work with Quicksilver Messenger Service, one of the leading bands in San Francisco’s psychedelic heyday, died June 29 in a hospital in Woodland, Calif. He was 72.

His wife, Dara Love Duncan, confirmed the death. She said that he fell June 19, suffered a seizure and multiple cardiac arrests and never regained consciousness.

Duncan’s jazz-rooted improvisations and his intricate interplay with guitarist John Cipollina were crucial elements in Quicksilver Messenger Service’s eclectic chemistry. Although Cipollina, who died in 1989, was nominally the lead guitarist and Duncan played rhythm, they constantly traded and blurred those roles.

David Freiberg, the band’s original bassist, called Duncan Quicksilver’s “engine.”

“He was what was holding the band together,” Freiberg said in an interview. “If he was there, it would work. If he wasn’t, it wouldn’t.”

The Quicksilver Messenger Service album “Happy Trails” (1969), largely recorded onstage, is a quintessential acid-rock artifact. The band starts out playing Bo Diddley’s “Who Do You Love” and jams its way far afield: through modal vamps, guitar dialogues, feedback, call-and-response audience screams and more.

“In Quicksilver we played a lot of old blues and folk tunes and improvised,” Duncan told ethnomusicologist Craig Morrison in 2001. “We had our own arrangements and had open places where we would just wail. ‘Cause we were so stoned all the time anyway, on acid and everything else, that sitting down and just playing one tune for three or four hours was nothin’.”

Duncan was born Sept. 4, 1946, in San Diego. In a 2007 interview, he said that he was originally named Eugene Duncan Jr., after his half-Cherokee, half-Scottish father, and that his mother, Jeraline Smith, was a Pawnee. He was adopted at birth by a Cherokee family: Kermit Grubb, who ran a secondhand store, and his wife, Pauline (Weaver) Grubb, a homemaker. They named him Gary Ray Grubb.

He played guitar as a teenager in rock and R&B bands in Northern California. He called himself Gary Duncan when he and drummer Greg Elmore agreed to join a band in 1965 that Cipollina intended to form with singer and songwriter Chet Powers, then going by the name Dino Valenti.

But Powers was imprisoned on marijuana charges before the band ever performed. Duncan became one of the band’s lead singers, in a lineup that also included Jim Murray on guitar and Freiberg on bass.

The new group took on the name Quicksilver Messenger Service, which was derived from astrology. Four members were born under signs ruled by the planet Mercury, who is the ancient Roman messenger god. The element mercury is the liquid metal also known as quicksilver.

Quicksilver soon gathered a following in San Francisco, regularly playing on weekends at the Avalon Ballroom and the Fillmore alongside Jefferson Airplane and the Grateful Dead.

In 1965, Duncan married Shelley Eidson, who later (as Shelley Duncan) wrote a book, “My Husband the Rock Star,” about their 10 years together. They had two children, Jesse and Heather, before divorcing. They survive him, as do Dara Duncan and their three sons, Miles, Michael and Thomas, five grandchildren; and a great-granddaughter.

After performing at the Monterey Pop Festival in 1967, Quicksilver was signed by Capitol Records, which the next year released its debut album, named after the band. The group began touring nationally. For most of 1969 keyboardist Nicky Hopkins replaced Duncan, and the band recorded the album “Shady Grove” without him.

Quicksilver was transformed when Duncan rejoined it. The band traveled to Hawaii and recorded the 1970 album “Just for Love.” The album cover’s design was inspired by the sugarcane and pineapple fields in Haleiwa.

The band also performed a free concert at the Kapiolani bandstand on March 16, 1970, entertaining about 1,000 fans and irritating many residents living near the park.

Valenti took over as songwriter and lead singer, with more pop-soul-flavored songs. Under the pseudonym Jesse Oris Farrow, he wrote Quicksilver’s most widely heard songs, “Fresh Air” and “What About Me,” both released in 1970. Quicksilver’s lineup went through multiple changes before a final 1970s album, “Solid Silver,” in 1975.

Quicksilver dissolved as the 1970s ended, and Duncan went to work as a longshoreman.

“With him it was like love-hate with the music business,” Dara Duncan said. “Sometimes he’d take a sabbatical.”

In the early 1980s, he built his own recording studio. He reclaimed the Quicksilver name in 1986 to release an album of synthesizer- driven 1980s-style pop, “Peace by Piece.” During the 1980s and ’90s he made extensive studio recordings, which the Global Recording Artists label has been compiling into albums. A new one, “Anima Mundi,” is forthcoming, said Karl Anderson, the label owner.

In 2004, Crawfish of Love, a jam band from Jacksonville, Fla., contacted Duncan online and made an album with him, “Snake Language.”

Interest in early Quicksilver was rekindled by a flood of live recordings of the original band. In 2006 Freiberg persuaded Duncan to join him on tour with Freiberg’s other band, Jefferson Starship. Performing together again as Quicksilver Messenger Service, Duncan and Freiberg shared concert bills with Jefferson Starship through the late 2000s. Duncan retired after those tours.

“He told me,” Anderson said, “‘I put it all down on tape. I did it all.’”