The effect of the converting enzyme inhibitor captopril (5 mg (kg bwt)-1) on systemic and renal haemodynamics has been studied in conscious rats in which a progressive hypertension has been induced by progressive aortic ligation (AL) between the renal arteries, and in a sham-operated (SO) group. Cardiac output (CO), organ blood flow and vascular resistances have been measured using radioactive microsphaeres. Captopril infusion caused increases in CO in both groups of rats, but the increase was higher in SO (9.2 +/- 0.7%) than in AL rats (5.2 +/- 0.6%; P less than 0.005). Plasma renin concentrations were similar in both groups but increased more in AL (10.3 microIU +/- 1.0) than in SO (5.81 microIU +/- 0.62; P less than 0.05) after captopril. Captopril induced also a larger decrease in arterial pressure (36 +/- 4 mmHg), and of the pressure gradient across the stenosis (19 +/- 3) mmHg in AL than in SO rats (5.6 +/- 1.4 and 1.1 +/- 1.3 mmHg, P less than 0.005 for both cases). Vascular resistance of the kidney above the ligature decreased more in AL than in SO rats, but this difference was not observed in the other kidney. From these data it can be concluded that captopril has an acute hypotensive effect despite the normal renin levels of this model of chronic hypertension. In addition, blood flow to the high-pressure perfused kidney seems to be dependent on the increased renin production by the contralateral kidney.