The movie also features Rafelson, Nicholson, Jon Andersen, and Dennis Hopper in interesting cameos during the Columbia-Screen Gems Studio Club café sequence; other cameos include Helena Kallianiotes (as a belly dancer in the “Can You Dig It” number; she would go on to star as Palm Apodaca in Five Easy Pieces [Columbia, 1970], another Bob Rafelson-Jack Nicholson vehicle) and wrestler Tiger Joe Marsh (as a security guard). Nicholson and Hopper would, of course, later be featured in Easy Rider (Columbia, 1969), which was executive-produced by HEAD executive producer Bert Schneider, and also featured HEAD choreographer Toni Basil.
You notice that HEAD has no opening credit sequence.