The long-term goal of this research is to provide greater insight into the role of CNS serotonin in human behavior. In the past few years, compelling evidence has been adduced to implicate serotonin in the mechanisms of action of potent hallucinogenic drugs, certain aspects of sleep, and in a variety of clinical psycho- and neuro-pathological disorders. The present research program seeks to clarify the functional role of CNS serotonin through animal experiments which analyze the basic component of the system, serotonin-containing neurons of the raphe nuclei, in freely moving cats.