1. Hemoglobin concentration, blood volume, and erythrocyte radioiron uptake were measured in rats subjected to acute protein deficiency.
2. Removal of protein from the diet was followed promptly by hemoconcentration, diminution in blood volume, and drastic reduction in erythropoiesis. These changes were reversible, after 35 days, upon addition of protein to the diet.
3. Protein intake is more essential for maintenance of normal erythropoiesis than is total caloric intake.
4. The data suggest that hemoglobin concentration within the vascular system is more important than red cell volume in regulating erythropoietic rate.