The circadian periodicity of plasma renin activity (PRA) during continuous recumbency was determined five times in a patient with primary hyperaldosteronism. All curves were parallel with one another. There were significant elevations of PRA at 1600-2000 h, falls at 2400 h, and peaks at 0200-0800 h. This periodicity was uninfluenced by changes in baseline PRA, serum potassium concentration, salt intake, plasma volume, blood pressure, or mineralocorticoid effect. It persisted despite inability of the patient to sleep on two nights. It is suggested that the observed periodicity of renin is a true endogenous diurnal rhythm of possible central nervous system origin.