The blood supply of healing undisplaced closed fractures of the radius and tibia in adult mongrel dogs was studied by means of microangiograms and histological preparations. The microangiograms were prepared from decalcified longitudinal slices across the fracture site, one millimeter in thickness. The corresponding histological preparations were made from the same slices of tissue. The vascular pattern at the site of fracture, at intervals from one day to eight weeks after fracture, was compared with the normal vascular pattern. Immediately after fracture there was a marked opening up of the existing arterial tree. With the advancement of healing, both medullary and periosteal circulations increased greatly by the development of new blood vessels; but the medullary arterial system, when intact, played the major role in the supply of blood to the uniting callus and in the revascularization of the necrotic cortex at the fracture site. The ascendency of the medullary blood supply increased as healing progressed. In the dog the normal configuration of the blood supply of the compacta is similar to that in man; the inner two-thirds of the cortex is supplied by branches of the nutrient artery. Our experimental findings in the dog support clinical experience. The periosteal circulation of long bones can take over when the medullary circulation has been interrupted by displacement of the fracture fragments or by surgery—as will be described in subsequent reports of experiments in progress. However, the medullary arterial supply, when available, dominates the vascular picture in rapid fracture healing.