The relationship of serum erythropoietin (Ep) levels to hematocrit and glomerular (GFR) filtration rate was evaluated in patients with chronic renal disease. The Ep level was measured by radioimmunoassay in 119 blood sampies from 48 patients obtained over a period of up to 5 years. Hematocrit values correlated significantly with the GFR, but serum Ep levels did not change with a decline in the GFR. Significant anemia was noted only when the GFR fell below 20 ml/min/1.73 m². Episodes of spontaneous acute hypoxic stress were observed in six patients with chronic renal failure. Serum Ep levels obtained during these episodes (mean±SEM: 273±76 mU/ml) were tenfold higher than Ep levels during stable steady-state chronic renal failure (26±6 mU/ml), even through Ep levels were inappropriately low for the degree of anemia in the stable state. Our findings suggest that the tissue oxygenation-Ep-hematocrit feedback mechanism operates at a lower set point in patients with chronic renal failure in comparison with normal subjects.