A detailed, metabolic investigation has been conducted in 3 cases of periodic catatonia over a period of 2-3 years.Case 1, a woman whose illness dated back to 1939 and who for 3 years prior to the present study had experienced recurrent violent excitements, was found to have changes in basal metabolic rate and nitrogen excretion correlating with her phasic changes. The disturbance has been brought under control by treatment with thyroid hormone and she has returned to work.Case 2, a man whose periodic psychotic excitements date back to 1936, was found to have a relative deficiency in 17-ketosteroid excretion with normal corticoid output. ACTH caused a normal adrenal response only in the corticoid fraction. This patient also had a low basal rate of oxygen utilization during interval periods and thyroid medication has brought his illness under control.Case 3, a woman with recurrent attacks of stupor since 1945, was found to have a relative excess of 17-ketosteroids in her urine with marked increases just prior to the onset of the deep retardation. The adrenal response to ACTH was relatively good, with a somewhat greater increase in corticoids than in 17-ketosteroids. Preliminary trials with cortisone improved her mental state and may provide specific, corrective therapy in this case.The possible significance of the low rate of oxygen utilization and its relationship to a state of hormonal imbalance are discussed.The importance of a careful determination of the hormonal pattern is stressed, if the causes of certain types of mental illness are to be understood and the success of specific, corrective therapy is to be evaluated.