1. The relation between the urinary excretion of inorganic phosphate and sodium was studied in anaesthetized dogs subjected to acute unilateral increases of ureteral back-pressure while receiving infusions of iso-osmotic sodium chloride. Under these circumstances modest increases of ureteral back-pressure, +14 to +23 cmH2O, were associated with relatively small changes of glomerular filtration rate from control values (−12.7 to +8.2%).
2. Increased ureteral back-pressure caused closely proportionate decreases of urinary phosphate and sodium excretion regardless of whether glomerular filtration rate increased, decreased or remained unchanged. When glomerular filtration rate increased or remained stable, the decreases of phosphate and sodium excretion were attributable to closely proportionate increases of tubular reabsorption of sodium and of phosphate. The increased tubular reabsorption of phosphate may be causally related to the increased tubular reabsorption of sodium.