The present study aimed to assess the relationship between erythropoietin (EPO) secretion and hyperoxemia in uremic patients. In 19 patients with chronic renal failure (10 were hemodialyzed and 9 were not dialyzed) and in 13 healthy subjects plasma erythropoietin levels were assessed during 6 h of air breathing and a second time during 2 h of pure oxygen breathing and during 4 h after discontinued oxygen breathing. Under basal conditions, uremic patients showed higher plasma erythropoietin levels (39.85 ± 5.86 mU/ml in hemodialyzed and 29.05 ± 4.94 mU/ml in nondialyzed patients) as compared with healthy controls (21.04 ± 1.77 mU/ml). Pure oxygen breathing was followed by a significant decline of plasma EPO levels both in patients with chronic renal failure and in the control group. However, this decline was significantly less marked and of longer duration in chronic renal failure patients than in healthy contro