• Biochemical and electrophysiological factors were studied longitudinally in a rapidly cycling manic-depressive patient. Slow changes in mood, motor activity, sleep, and urinary norepinephrine levels during the course of each depressed and manic episode are reported, as well as rapid alterations in many variables at the time of mood switch. Urinary concentrations of norepinephrine and its metabolite, 3-methoxy-4-hydroxyphenyl glycol (MHPG) were significantly lower in depression than in mania; norepinephrine but not MHPG excretion increased prior to the switch. We postulate that the slow behavioral and biological changes preceding switches in this patient are an important manifestation of the cyclic process in manic-depressive illness.