It has long been known that a number of drugs or chemicals are capable of inducing psychic disturbances. Recent reports on central nervous system effects of certain neurohumors and chemically related agents prompted us to investigate some humoral aspects of psychoses. Hofmann discovered that LSD-25 induces psychotic phenomena in man, a finding which was described in detail by Stoll (14). Amin and his coworkers (1) found to be present in the brain an agent which is related to LSD-25, and which was identified as serotonin (5-hydroxytryptamine). Woolley and Shaw (15) suggested that a deficiency of serotonin in the brain may cause schizophrenia. Goodman and Gilman (11) reported that in some people epinephrine produced psychic effects such as fear, anxiety, tenseness and restlessnesssymptoms which to us suggested a similarity to manifestations of the manic phase of manic-depressive psychosis. They also, delineating the psychic effects of amphetamine (and other epinephrine congeners), stated that in therapeutic doses these drugs cause elevation of mood, euphoria, elation, and increased motor and speech activity; and in large dosage cause dizziness, agitation, confusion, dysphoria, apprehension, delirium, fatigue, or depression. Again, we feel that these drug effects resemble manifestations of manic-depressive psychosis. In our earlier studies, we reported that serotonin has potent neurotropic properties (6). When this agent was administered to the dog brain via the internal carotid artery, transient hemiplegia developed. Small doses of serotonin were found to cause spastic paralyses, whereas large doses produced flaccid paralyses.