Wistar rats were made polycythemic by daily injections of massive numbers of red cells for one week. The injections were made into the peritoneal cavity and the Ht level rose to between 70 and 75%. This plethoric state resulted in suppressed erythropoiesis of the bone marrow. At this stage, a quantity of blood was drawn from the orbital sinuses with concomitant transfusion of a saline-glucose-blood serum mixture. This withdrawal-transfusion process took about 30 minutes and the Ht value decreased from 75 to 25%. Distinct proliferation of erythroblasts and basophilic erythroblasts was observed 6 to 9 hours after blood withdrawal. Polycythemic animals had many large blast-like cells having relatively large clear cytoplasm. In each of these cells, the nucleus had fine chromatin nets and an ambiguous nucleolus. At 3 hours after blood withdrawal proerythroblast like cells with basophilic cytoplasm had appeared, but there were no basophilic erythroblasts. Typical proerythroblasts having characteristic nuclear chromatin nets and deep blue cytoplasm were prominent after 6 to 9 hours. More mature, poly- and orthochromatic erythroblasts appeared between 9 and 12 hours after blood letting. At 72 hours after blood letting, the cytohistologic picture of the bone marrow of polycythemic animals had returned to nearly normal. Possible erythroid cell differentiation from precursors in the bone marrow and from circulating blood is discussed.