Control values of Na, Cl, and K for renal lymph, plasma and thoracic duct lymph were obtained from 30 anesthetized dogs. Renal lymph was found to have an average Na concentration of 162.1 [plpus or minus] 2.8 meq/1 11.3% higher than the average plasma value. The average Cl concentration of renal lymph was 140.2 [plpus or minus] 3.8 meq/l, 26.9% higher than the plasma Cl. The mean K concentrations for all 3 fluids were similar. Renal lymph in the normally functioning kidney is probably derived from 2 sources: capillary filtrate and distal tubular reabsorbate.