A JSM-U3 type scanning electron microscope was used to conduct stereoscopic and continuous observation and photography of the microvascular architecture of the rat femoral bone marrow by the injection replica method using methylmethacrylate. The nutrient artery passes through the nutrient foramen at an angle of 30 on the proximal and inner side of the femur to the bone marrow and bifurcates into ascending and descending branches. The ascending branch continues to run proximally, while the descending branch further divides into 3 immediately after the branching and all 3 bend and curve as they continue to proceed distally and eventually separate into numerous arterial branches. Approximately 1/3 of the bone marrow which is located near the nutrient foramen is linked to the arterial branch which has divided in the nutrient foramen. This arterial branch passes between the sinusoid and proceeds to a point comparatively close to the surface of the bone marrow where it separates into finer branches and flows into the sinusoid. It is considered that the distal two thirds of the bone marrow is connected to the arterial branches which divided from the descending branch. There are also branches which pass through the periosteum to the Haversian canal, enter the bone marrow and connect with the sinusoid. The opening of the arterial capillaries assumes various shapes such as gradual and steep funnellike to right angle orifices. The injection replica scanning electron microscope method is a highly effective means for elucidating microvascular architecture such as that of the bone marrow and it is felt the author was quite successful in clarifying the microvascular archtecture of the bone marrow.