Radiotherapy is an important mode of therapy for malignant tumors in the oro-mandibular area and the changes in vascularization which it causes are considered good indicators of both healing and the results of combined surgery and chemotherapy. The author studied the effects of radiotherapy on teeth and mandibular vascularization in 60 adult dogs. Cobalt irradiation of the lower right mandible was followed by intravascular injection of chloropercha to delineate the three-dimensional morphology of the changes. The histopathology, X-ray and macroscopic characteristics were also studied. The results were: (1) With 600 rad, fine capillaries around the roots of teeth increased. With 900-1,500 rad, the course of blood vessels became irregular. With 1,200 rad, there was a marked increase in capillary vessels which extended meanderingly. With 4,500 rad, the number of blood vessels was markedly decreased. Some vessels were irregular and unclear. (2) Vessels in the dental pulp showed mild meandering with 900 rad. They increased markedly with 1,200 rad and showed meandering. With 4,500 rad, the number of blood vessels decreased. (3) The histopathology showed that, with comparatively small amounts of irradiation, marked vascular disturbance accompanied by vascular dilation and edema had occurred. As the amount of irradiation increased, increased fibrosis of interstitial tissue, decreased numbers of blood vessels, and aseptic necrosis occurred, resulting finally in bone absorption and bone necrosis. (4) X-ray examination showed that mild constriction of the pulp cavity occurred. with 3,000 rad. This became marked with 4,500 rad and showed absorption. (5) Macroscopically, 900 rad caused mild edema in the cheek skin. With 3,000 rad, the oral cavity showed some ulceration. With 4,500 rad, necrosis of the gingiva occurred. (6) A certain correlation in the changes of blood vessels, X-ray findings, macroscopical, and pathohistological findings was evident.