In 19 patients with unilateral renal artery stenosis and subsequent renovascular hypertension plasma renin activity (PRA), plasma concentrations of atrial natriuretic peptide (ANP), erythropoietin (Epo), H+ and HCO3-, as well as pO2 and pCO2 were assessed in renal venous blood of the 'ischaemic' and normally perfused kidney, both in arterial blood and in the inferior vena cava distally from the orifices of the renal veins. PRA and ANP were significantly elevated in venous blood of the ischaemic kidney as compared with the normally perfused kidney. In contrast to PRA and ANP, plasma concentrations of Epo were similar in blood withdrawn at all vascular sites. pO2 and pCO2, as well as blood H+ and HCO3- concentrations in venous blood of the ischaemic kidney were of the same magnitude as of the normally perfused kidney. From the results presented in this paper it follows that (i) in contrast to plasma renin activity and ANP, unilateral renal 'ischaemia' does not influence plasma concentrations of Epo in renal venous blood, and (ii) chronic haemodynamic alterations do not seem to influence Epo secretion by the kidneys.