Specific alterations in the microvascular and connective tissue compartments of the hemopoietic microenvironment have been examined during erythropoietic regeneration and suppression in the murine spleen and bone marrow using in vivo microscopic and histochemical methods. The results have confirmed the concept of specific hemopoietic microenvironments and have demonstrated specific alterations in the microenvironment during erythropoietic stimulation and repression. Elevated erythropoiesis in the splenic red pulp is accompanied by an elevation in blood flow through the microvascular system. Both the linear velocity of flow and the number of sinusoids with blood flow in them increased significantly. In contrast, erythropoietic repression was accompanied by a decreased linear velocity of blood flow, as well as a marked increase in the amount of blood being stored in the splenic sinusoids. This also was the picture when diffuse granulopoiesis was present in the red pulp, or when granuloid or undifferentiated colonies were present. The chemical composition of the stroma in the spleen and bone marrow also varied during states of hemopoietic activity and, in addition, there were differences in the composition of the stroma between these two organs. In both organs, foci of early proliferating cells were enveloped by a coating of sulfated acid mucopolysaccharide. This coat persisted on cells in later stages of granulopoiesis but not on cells in the later stages of erythropoiesis. The latter were enveloped with a coating of neutral mucopolysaccharide. A tentative hypothesis to explain the mechanisms involved in producing these changes is discussed.