Bipolar manic-depressive illness is a chronic disease in which patients experience recurrent episodes of mania and depression. Patients often change from a nonverbal, retarded depression of many months' duration to a hyperactive, psychotic, manic condition during the switch. The time required for the switch from depression into mania varies from 5 minutes to a couple of days. Just before it happens, pateints experience marked insomnia and decreased rapid eye movement sleep. It is hypothesized that specific changes in brain monoamine metabolism precede the switch. Alterations in neurotransmitter metabolites, as measured in urine and cerebrospinal fluid, may precede and accompany it. The switch into mania can be precipitated by environmental stresses or by drugs that act by increasing functional brain monoamines. Drugs that reverse the manic state all share the common property of affecting biogenic amines. The switch into mania is viewed in the context of a longitudinal cyclic process and may be further studied with specific pharmacologic agents that block drug-induced maniclike states in man.