Studies were done of cell production by marrow in diffusion chambers implanted in the peritoneal cavity of rabbits subjected to various stimuli to hematopoiesis. In chambers in neutropenic hosts and in hosts injected with endotoxin, animals presumed to have an increased stimulus to granulopoiesis, there was increased production of granulocytes but there was also increased production of red cells. Although red cell production was decreased in chambers in polycythemic hosts, granulocyte production was not different from that in controls. Stimulation of erythropoiesis by erythropoietin injections or by exposure to hypoxia increased red cell production by marrow in the implanted diffusion chambers without diminishing granulopoiesis. Only in chambers in hosts made anemic by bleeding was there an increase in red cell production accompanied by a decrease in granulocyte production. In these anemic hosts induction of neutropenia led to an increase in granulopoiesis without any depression of erythropoiesis.