Peter Fonda was involved in a number of psychedelic films in the end of the 1960s, among them the classic Easy Rider and Psych Out, both of which featured the psychonauts-friendly young Jack Nicholson. The Trip (1967) is perhaps the most psyhedelically ambitious film of them all: it is perhaps the only film which is wholly dedicated to following one trip of one person for almost the whole duration of the film. Anyway you look at it, it is a unique cinematic adventure in psychedelia.
You can watch the entirety of the film on YouTube in the link above.
The Daily Psychedelic Video 