Altered States (1980)

This film surprisingly still packs a wallop -- what a head-trip it is.  Starring '80s lead actor William Hurt in his feature debut (he still pops up in indies, but is not the draw he once was) and directed by Ken Russell in the days when a studio would still bankroll him, this movie is based on a Paddy Chayevsky novel on the possibility of experiencing different levels of awareness and existence.  Hurt plays a researcher dabbling with sensory deprivation tanks and mind-altering drugs and goes further than his medical colleagues Bob Balaban and Charles Haid (always associated by me with Hill Street and not endocrinology) would advise, regressing at different stages to a murderous simian being and pure spirit (I think).  Cue wonderful psychedelic special effects that would have been more in tune with the earlier LSD eras of the '60s and '70s, but still mind-boggling to behold.  It's the kind of movie that the viewer must just accept as it stands, rather than trying to make head nor tail of it.  Apparently Mr. Chayevsky was so perturbed by Russell's interpretation of his book that he tried to get him removed from the helm and his name taken off the credits; the screenplay is therefore credited to a Sidney Aaron which is actually Mr. C's real name.