To investigate the influence of critical illness on plasma renin activity and aldosterone levels and to examine potential inhibitory effects of dopamine therapy on aldosterone responsiveness, we measured plasma renin activity, and potassium and creatinine in serum, as well as the responses of aldosterone, cortisol and prolactin levels to TRH 200 micrograms i.v. + Synacthen 0.25 mg i.v. in 63 unselected, critically ill patients (32 females, 31 males, aged 18-84 years). Of the patients 19 received dopamine treatment (3-13 micrograms/kg/min i.v.); 21 of the patients died in the further course of their disease. Plasma renin activity was increased in 66.7% of the patients and aldosterone levels were elevated in 90.5% of the patients. There were correlations (P less than 0.05) of lethality with plasma renin activity and cortisol levels and correlations (P less than 0.01) of aldosterone concentrations with plasma renin activity and cortisol levels. Whereas dopamine treatment had no inhibitory effect on aldosterone levels before and after stimulation, prolactin stimulation was decreased in dopamine-treated patients. Thus, dopamine does not generally lose its potency of hormone inhibition in critically ill patients, but has no influence on the secondary aldosteronism developing regularly in the early phase of critical illness, which is apparently mainly due to the stimulatory effect of ACTH (or ACTH-related pituitary peptides) and is considered an epiphenomen of the stress mechanisms acting upon the patients in this condition.