Forty dogs were divided into five groups: (A) control, (B) 1000 rads, (C) 2400 rads, (D) 3000 rads and (E) 3600 rads. The medial aspect of one pelvic limb was irradiated through a port measuring 12 × 8 cm. Prior to irradiation and at intervals after, the pelvic limb lymphatics and popliteal lymph node were visualized by an indirect technique of lymphangiography. The afferent and efferent lymphatic vessels in the control and treated animals remained essentially unchanged except in four animals. These four animals, one in the 2400 rad group and three in the 3600 rad group, exhibited rupture of the afferent lymphatic vessels at the junction of irradiated and nonirradiated tissues several months after treatment. All treated animals showed a marked permanent decrease in size of lymph nodes several months after treatment. There was no evidence of lymphatic obstruction in any group. The authors conclude that ionizing irradiation appears to have little effect on lymphatic vessels. As reported previously, the lymph node undergoes fibrosis without obstruction.