10% saline solution was infused in the dogs by constant infusion and single injection, and the changes in plasma Na concentration and urinary Na excretion rate were followed. The responses of animal to the two types of saline loading were analyzed in terms of a single capacitive system and a two compartment model. The two compartment model was con-structed by one extracellular and another intracellular compartments connected by leak at the cellular membrane and Na pump at the same site and by modulation of renal reabsorption of Na by the intracellular Na concentration. In the dog kidney, the experimental results were accounted for better by the threshold theory of the Na reabsorption than by the constant fractional reabsorption theory. Advantages and incompletenesses in the present model were discussed.