Protein starvation nearly arrested erythropoiesis in rats, and the red cell mass decreased during a period of 32 days from 4.0 to 2.5 ml. Protein-starved rats, injected daily with 1.8 units of rabbit erythropoietin, had reticulocyte counts within the normal range, and their cell mass increased during the same period to 4.26 ml., as compared to 4.52 ml. in normal controls. Red cell indices, Price-Jones curve and osmotic fragility were normal on blood obtained from erythropoietin-treated groups. Differential counts on their erythroid marrow composition were not significantly different from those in normal rats. It is concluded therefore that daily injection of 1.8 units of rabbit erythropoietin induced, over a period of 32 days, a steady state erythropoiesis which, on the basis of the parameters studied, could not be distinguished from that in normal rats. No evidence of a shortened lifespan of cells formed in response to erythropoietin was found after either random (Cr51) or cohort (Fe59) labeling. Random labeled (Cr51) cells from untreated protein-starved rats had significantly shorter chromate survival times than cells from normal or erythropoietin-treated rats. The difference is attributed to the altered age distribution in their red cell population.