Twenty-four-hour urine specimens were collected daily from a rapidly cycling manic-depressive patient throughout a 3-month hospitalization, and were analyzed for volume, osmolality, creatinine, 17-OHCS, and VMA. Mean 17-OHCS excretion was significantly greater during depression than during hypomania, but only in the latter half of hospitalization, after the patient had become acclimatized to the hospital. Mean VMA excretion was significantly increased only during the period of hypomania when the patient was physically active. The 17-OHCS data suggest that interaction between stressful environmental stimuli and intrapsychic ego-defense strength determines the adrenal-cortical activity level during any given phase of the manic-depressive cycle. The VMA data are in accord with previous studies of catecholamine excretion in cyclic affect disorders.AN RECENT YEARS many psychoendocrine studies have utilized a longitudinal sampling methodology in which biochemical measures are made repeatedly on a small number of patients. Longitudinal sampling, in contrast to crosssectional methodology, permits intraindividual comparison of endocrine measures during different phases of many types of stress states. For longitudinal studies, manic-depressive illness