The vascular bed of mammalian red bone marrow has been studied. The walls of sinusoids are morphologically simple structures consisting of flattened reticular cells which make simple contacts with the processes of neighboring cells. Often gaps in the wall are visible. No basement membrane underlies the reticular cells, and there is hardly any stromal support. These features are presumed to allow sinusoids a functional flexibility absent in most other blood vessels. Colloidal thorium dioxide particles introduced into the blood stream become dispersed throughout the marrow parenchyma within 10 minutes, indicating the functional patency of the sinusoidal walls. The extent and speed of this scattering indicates that the parenchymal cells are actively in motion at all times. “Intersinusoidal capillaries” are not to be found in red marrow, even after experimentally indroducing an aplastic anemia.