Plasma renin activity (PRA) and plasma concentrations of angiotensin II (AngII), atrial natriuretic peptide (ANP) and arginine vasopressin (AVP) were determined in the abdominal aorta and the renal veins before and 1 h after peroral ingestion of captopril 25 mg in 29 patients with arterial hypertension and unilateral renal artery stenosis or occlusion, in order to study the effect of ACE inhibition on single-kidney extraction ratio (ER) of PRA, AngII, ANP, and AVP, and on renal vein renin ratio (RVRR). PRA was increased, AngII and ANP were reduced, and AVP unchanged after captopril. On the affected side the negative ER or PRA (-1.03) and AngII (-0.28) and the positive ER of ANP (0.25) and AVP (0.14) were not significantly changed by captopril. On the non-affected side ER of AngII and ER of ANP were significantly reduced (ER of AngII, 0.41-0.00, ER of ANP, 0.29-0.17), but ER of PRA and ER of AVP were unchanged. RVRR was not significantly changed by captopril. RVRR was greater than 1.5 in 79% of the patients before captopril, in 82% after captopril, and in 93% either before or after captopril. It is concluded that captopril reduces ER of AngII and ER of ANP on the non-affected side but not on the affected side in unilateral renal artery disease with hypertension, and that the use of RVRR both before and after captopril improves the predictive value of RVRR with regard to the diagnosis of unilateral renal artery disease.