Roger Corman also took LSD before starting the film, figuring he couldn't make a film about LSD without trying it himself. He had a good experience, and had to ask others what a "bad trip" was like in order to incorporate it into the film.
The band in the club near the beginning is the International Submarine Band, featuring Gram Parsons on vocals. However, their early country-rock sounds were removed and the psychedelic sounds of The Electric Flag were dubbed into the film.
The film was rejected for a UK certificate 4 times by the BBFC (in 1967, 1971, 1980 and 1988) as it was considered an advertisement for the use of LSD. The film was shown uncut on the FilmFour satellite channel in 2002 and finally given a fully uncut UK DVD certificate in January 2004, almost 37 years after its first BBFC submission.
Bruce Dern's line "Turn off your mind, relax, and float downstream" is from The Beatles' song "Tomorrow Never Knows", which John Lennon wrote as a summation of his reading of "The Psychedelic Experience", which in turn was adapted from "The Tibetan Book of the Dead".
According to Peter Fonda, he got visibly aroused while filming his nude scene with Salli Sachse. "Salli just clamped her legs together to keep it out of sight," Fonda said, and that director Roger Corman continued to film regardless.