Bone marrow sinuses of young rats were examined under the scanning (SEM) and transmission electron microscopes (TEM). Marrow sinus wall was composed of three layers: an inner or luminal endothelium, an outer or adventitial cell layer, and a basal lamina in between. The luminal surface of the endothelial cells was quite smooth and showed some fenestrations, which could be divided into two types according to their size. One was represented by larger fenestrations (1-3 mum in diameter) which were presumed to be formed transiently at the site of blood cell migration, while the other by small pores (0.1 mum) grouped into a cribriform area. The adventitial cells showed a discontinuous layer in the TEM. Under the SEM, the discontinuity corresponded to the spaces formed between the cytoplasmic attenuations of the cells. Blood cell migration from the extravascular hemopoietic tissue into the sinus lumen was numerously observed. The migration occurred not through an intercellular gap, but through the larger intracellular fenestration of the endothelial lining mentioned above. A number of megakaryocytes were identified by their bulky cytoplasm in the parenchyme. Figures suggesting the sequence of platelet liberation from this cell could be demonstrated. First, the megakaryocyte extended its peripheral cytoplasmic processes into the sinus through endothelial fenestrations. The processes, being conspicuously extended, became periodically constricted. Finally, platelets were believed to be produced by separation at the constricted portions and liberated to circulation. The occurrence of a few endothelial fenestrations apparently unassociated with blood cell migration may possibly be ascribed to detachment of a blood cells due to vascular perfusion. The functional significance of the adventitial cell was discussed in association with blood cell migration.