In normal supine man, a marked circadian rhythm was found in plasma renin activity (PRA) and in plasma aldosterone concentration, with highest levels in the morning and lowest in the evening. Short-term suppression of ACTH by dexamethasone did not eliminate the circadian variation of plasma aldosterone. In patients with a transplanted and thus denervated kidney, no such rhythm of PRA could be observed. From our data it is suggested that the normal circadian rhythm of PRA depends on intact renal innervation and is controlled by the central nervous system.