In this study we examined erythropoiesis in the bone marrow and spleen of 9 to 12-day-old neonatal rats suckled by bleeding-induced anemic mothers. Quantitative evaluations of the spleen revealed a significant decrease in total numbers of nucleated RBC/mg spleen in 11-day-old pups nursed by the anemic mother which returned to control values by day 12. A significant reduction in total numbers of nucleated RBC/mg marrow was seen in both 11 and 12-day-old pups of anemic mothers when compared to control values. These results suggest that: 1. Ep is transmitted to suckling rats via the maternal milk; 2. Ep in the neonate exerts its influence predominantly at the level of the differentiated erythroid compartment probably by causing a shortening of the mean transit time of the proliferating erythroblast compartment and/or by decrease in the maturation time of the nonproliferating orthochromatic and reticulocyte compartments; 3. the regulation of erythropoiesis in the neonate differs from that in the adult.