Trailer for the 1968 movie Psych-Out, starring Jack Nicholson, Bruce Dern, Susan Strasberg, and Dean Stockwell.
From Wikipedia, which has a thorough plot synopsis:
Psych-Out is a 1968 feature film about hippies, psychedelic music, and recreational drugs, produced and released by American International Pictures. Originally scripted as The Love Children, the title when tested caused people to think it was about bastards, so Samuel Z. Arkoff came up with the ultimate title based on a recent successful reissue of Psycho.
(via PCL Linkdump)