Plasma cortisol and renin were estimated in 1 h intervals, plasma aldosterone, angiotensinogen and angiotensinases in 3 h intervals over periods of 24 h in six normal volunteers (age 20-26) under control conditions and subsequently under suppression of ACTH release by dexamethasone. Highest cortisol levels were found around 7 a.m., minimum levels between 9 p.m. and 1 a.m. Dexamethasone reduced cortisol to constantly low concentrations. Aldosterone was highest around 4 a.m. under control conditions and under dexamethasone, and showed lowest concentrations between 4 and 10 p.m. There were no significant differences between mean aldosterone concentrations at corresponding time points of the control and the dexamethasone period. Similar to aldosterone, renen showed peak values around 4 a.m. All mean values at corresponding time points between 7 a.m. and 11 p.m. and the 24 hour mean values of each subject were significantly increased under the influence of dexamethasone. No evidence could be achieved for the existence of circadian rhythms of angiotensinogen and angiotensinases. Dexamethasone did not cause significant changes of these parameters.