The effects of captopril 450 mg/day for 4 weeks on blood pressure, heart rate, cardiac output and extracellular fluid volume were compared in severe, often drug-resistant hypertension (n = 23), mild to moderate hypertension associated with renal artery stenosis (n = 10) and mild to moderate essential hypertension (n = 20). Plasma renin in the three groups was 52 +/- 19, 58 +/- 17 and 20 +/- 4 microU/ml (mean +/- SEM), respectively. Blood pressure fell by 18 +/- 4%, 21 +/- 2% and 18 +/- 1%. The pressure drop was mainly due to a fall in peripheral vascular resistance. Addition of the diuretic hydrochlorothiazide (25-100 mg/day) caused a further fall in resistance. Despite the vasodilator effect of captopril, reflex cardiostimulation and reactive fluid retention were not observed. In severe hypertension, captopril alone was more effective in lowering blood pressure than combined diuretic-betablocker-vasodilator therapy. Moreover, cardiac output in these patients was higher and resistance was lower after captopril than during combined treatment. Thus, captopril was capable of normalising the abnormal haemodynamic state in patients with essential hypertension, and in hypertension associated with renal artery stenosis. Despite marked differences in pre-treatment plasma renin, the effects of captopril on systemic haemodynamics were similar in all the patients.