PART I of the present investigation reports the behavioral analysis of a patient with regular 48-hour manic-depressive cycles. These cycles persisted with clock-like regularity over a twoyear period and afforded the researchers a unique opportunity to observe and describe manic-depressive cycles. Part 2 of this investigation will report biochemical correlates of the behavioral changes and will offer a theoretical formulation of the relationship between the observed behavior and the biochemical determinations. The intensive study of a single patient may provide information unobtainable in the study of large numbers of individuals and may help in the formulation of theoretical models which will have application beyond the single patient. This patient in the present study who will be referred to as Mrs. J had regular 48-hour cycles of mania and depression. On the depressed day the patient was slowed down, guilty, depressed, and sad-looking, while on the