Moderate increases of ureteral back pressure usually cause decreases of glomerular filtration rate and even greater decreases of sodium excretion. It has been assumed previously that increased ureteral back pressure does not enhance renal tubular sodium reabsorption directly and that the decreases of sodium excretion are caused by the decreases of glomerular filtration rate. In the experiments reported here, the effect of increased ureteral back pressure on urinary sodium excretion was studied in dogs in which changes of filtration rate were minimized by infusing saline while ureteral back-pressure was increased. When ureteral back pressure was increased on one side by 10-23 cm of water, the inulin clearance of the experimental kidney decreased by only 3-12% in 21 experiments, did not change significantly (+/-2%) in eight experiments, and increased by 3-8% in seven experiments. The sodium excretion of the experimental kidney decreased in all experiments regardless of whether its inulin clearance increased, decreased, or was unchanged from control values. When the inulin clearance of the experimental kidney increased or remained unchanged during increased ureteral back pressure, its reabsorption of sodium increased more than could be accounted for by the increase of filtered sodium. When the inulin clearance of the experimental kidney decreased during increased ureteral back pressure, its reabsorption of sodium decreased less than could be accounted for by the decrease of filtered sodium.Therefore, the effect of increased ureteral back pressure to decrease urinary sodium excretion is caused in part by increased tubular reabsorption of sodium.