Following thoracic duct obstruction in dogs the phenomenon of lymph reflux was studied by lymphography of the abdomino-visceral structures. Obstruction was produced by retrograde injection of Sotradecol 3% into the terminal portion of the thoracic duct. At intervals of 3 days, 1 week, 2 weeks and 4 weeks thereafter visceral lymphography was performed via the peripheral lymphatics of the small bowel. The description of the alterations in lymph flow was based largely on the lymphograms, plus gross autopsy findings in some dogs. In the period following thoracic duct obstruction when lymphatico-venous communications were still not functioning, reflux occurred into the hepatic lymphatics along the portal vein. After development of collateral channels to the general circulation, reflux readily occurred into the parietal lumbar lymphatic system. By 2 weeks after thoracic duct obstruction lymph flow was established from the viscera into the inferior vena cava via parietal lumbar lymphatics. In this situation of high intralymphatic pressure, lymphatic valves were not sufficiently strong to prevent backflow. After one occurrence of retrograde flow, backflow could then take place without difficulty; however, a high resistance to backflow is shown by the lymph nodes. There was no opacification of visceral nodes despite extensive reflux in this region. There was no evidence of reflux into the peritoneal cavity or into the lymphatics of the kidney, or intestine, presumably because of the barrier function of nodes to reverse flow. We believe that the loss of the lymph nodes as barriers to backflow permits chylous reflux to occur.