This study assessed the roles of expectancy and autonomic arousal in inducing psychedelic flashbacks under conditions of mild sensory deprivation. Subjects with and without previous flashbacks participated. During the first experimental session, subjects were given a drug which they were told would induce flashbacks, and during the second, a drug which was alleged to induce only autonomic arousal. Ephedrine sulfate (causing autonomic changes) or placebo actually was given in counterbalanced order. Subjects of both groups experienced many more psychedelic sensations when they expected flashbacks. Drug effect was nonsignificant. These findings support the view that flashbacks are not "caused" by psychedelic drugs, but by the tendency of some drug users to mislabel and selectively attend to relevant aspects of naturally occurring altered states of consciousness.