Using specific radioimmunoassays for aldosteronc (Aldo) and corticosterone (CPD B), the serum levels of these 2 mincralocorticoids were measured daily during 2 consecutive cycles in 5 normally menstruating subjects. Two subjects suffered from cyclic edema and premenstrual tension, respectively. In each patient the first cycle served as control, and the subjects received dexamethasone, 0.5 mg, 2 to 4 times daily during the second cycle. Mean scrum CPD E levels were higher in the luteal phase than in the fullicular phase in the control cycle of all subjects. Dexamethasone treatment decreased peripheral CPD B markedly, with levels -1 to 30 times lower. Serum Aldo was significantly higher in the luteal phase of the control cycle than in the follicular phase in 4 of 5 subjects. The 2 subjects with problems of water retention had significantly higher Aldo levels than the normal women studied. Dexamethasone treatment significantly lowered Aldo levels in these 2 subjects, whereas this treatment caused an increase in peripheral Aldo in 2 of the 3 normal subjects.