MATERIALS AND METHODS Blood was collected in ACD solution from a patient with methemoglobinemia due to hereditary deficiency of NADH-methemoglohin reductase8 and stored at 4 C. until use. Blood from a normal individual was obtained on the same day and similarly stored. The mice used in these experiments were 10 to 12 weeks old CF1 females weighing approximately 22 Cm.

Red cells were separated from enzyme deficient blood by centrifugation, washed with saline, and exposed for 20 minutes to an equal volume of a 1 per cent sodium nitrite solution at 5 C. Afterwards the red cells were washed ten times with cold isotonic saline to remove all excess nitrite. These cells contained 100 per cent methemoglobin and had no oxygen-carrying capacity. Normal human red cells stored for the same time were treated similarly except that they were not exposed to nitrite. Erythropoietin in plasma of heparinized blood was assayed according to the method of DeCowin et al. 9 by measuring 59Fe uptake into the hemoglobin of newly formed red cells of plethoric mice. Hypoxia was produced by placing mice into a 3’X 2’x 2’steel chamber evacuated by a vacuum pump and maintained at a pressure equivalent to 0.4 atmospheres. The mice were maintained in this hypoxic atmosphere for four and one-half hours. The red cell mass was measured 15 minutes after the intravenous injection of isologous red cells labeled in vivo with 59Fe. Red cell mass(RCM) was calculated according to