Water diuresis was induced in dogs, under chloralose anesthesia, by hypotonic volume expansion of 5 to 10% body weight. The average diuretic response was 12.5 ml/hr/kg body weight with a urine osmolality of 175 mOsm/L. Renal hilar lymph flow and thoracic duct lymph flow increased significantly by an average of 125% and 150% respectively. The Na concentration in plasma and lymph fell by approximately 7.0 mEq/L whereas that for Cl was relatively unaffected. Control lymph to plasma ratios of approximately 1.050 for Na and 1.130 for Cl were not significantly altered even when plasma concentrations fell significantly. Urea, glucose and K concentrations were little affected. The results are consistent with the hypothesis that renal hilar lymph reflects in part the composition of the outer medullary interstitium which is directly influenced by the electrolyte pump in the ascending thick limb of Henle.