Methylphenidate (Ritalin) hydrochloride was administered intravenously to schizophrenic, manic, and depressed patients during the active phase of their illness and upon recovery and to normal. Methylphenidate was found to activate psychotic symptoms in schizophrenics during the active phase of their illness. It failed to produce this effect in the same patients after remission had occurred. It failed to produce a psychotic reaction in most patients with mania and all normal or depressed patients.