Plasma levels of erythropoietin (EPO) were estimated in 14 patients with noninflammatory acute renal failure during the oliguric anuric and polyuric phase respectively. During the anuric oliguric phase plasma EPO levels were 4 times higher than in healthy subjects, while during the polyuric phase normal plasma levels were found in spite of the presence of anaemia. Results presented in this paper suggest presence of an abnormal feedback between EPO secretion and degree of anaemia in patients with acute noninflammatory renal failure both during the anuric/oliguric and polyuric phase respectively. Abnormal EPO secretion does not seem to be the only or even dominant factor involved in the pathogenesis of anaemia in patients with noninflammatory acute renal failure.