The effects of irradiation upon the functional integrity of lymph nodes were studied in the isolated cervical region of the dog. After 6000 rads were applied in one dose plan, the measure of lymph flow was determined by the migration of radiogold from the larynx to the associated primary lymphatic terminals at intervals of 2, 4, and 12 weeks, and six months post-irradiation. Interference with lymph flow was identified by radionuclide stasis in the injection depot; alteration in the direction of flow was identified by relative concentration of the tracer item in the associated nodal terminals. The lymph flow was minimally distorted by irradiation in the test period unless secondary damage to the associated lymphatic channels intervened. The relative migration was consistent with the normal data in 52%, depressed in 13% and increased in 35% of the cases studied. Major obstruction and rerouting of lymph flow contralaterally was evident in 19% of the subjects due to fibrosis secondary to local infection.