Plasma renin (PRC) and angiotensin I (pANG I) and II (pANG II) concentrations were determined at renal vein catheterization in 38 hypertensive patients suspected of renal or renovascular aetiology. Veno-arterial differences in pANG I across the affected kidney in patients with lateralization of the renin secretion indicated release of angiotensin I (ANG I) in considerable amounts. Veno-arterial differences in pANG II of around zero indicated that generation and elimination of angiotensin II (ANG II) were equal in that kidney. Across the contralateral kidney the veno-arterial differences in PRC and pANG II were both close to zero, while negative differences in pANG II indicated the removal of ANG II. In patients without lateralization of the renin secretion the veno-arterial differences in pANG I were close to zero or positive, those of pANG II being close to zero or negative. In 14 of the 38 patients, data were obtained either during maintenance treatment with captopril or after a single dose of this converting enzyme inhibitor. Systemic PRC and pANG I values were extremely high and pANG II values low. The pANG I gradient across the affected kidney was further increased compared with pre-captopril levels, whereas the contralateral kidney still eliminated ANG I.