The effect of unilateral ureteral obstruction (UUO) of 48 h duration on phosphate excretion was studied in dogs. After the relief of UUO, the fraction of filtered phosphate excreted (FEP) was markedly reduced while that of sodium (FENa) was significantly increased as compared to the normal kidney. Extracellular fluid volume expansion (ECVE) in dogs with intact parathyroid glands and in those with thyroparathyroidectomy produced marked phosphaturia from both kidneys but significantly less from the obstructed one. Infusion of parathyroid extract (PTE) to dogs with intact parathyroid glands caused similar effects to those seen with ECVE; PTE infusion also caused a significant but equal rise in urinary cyclic AMP. In other groups of animals, acute hypoperfusion of the left kidney was produced by constriction of the aorta between the origin of the renal arteries. The patterns of FEP noted from both kidneys in these dogs during ECVE and infusion of PTE were qualitatively similar to those seen in dogs with UUO. The results indicate that (1) there is a dissociation between the renal handling of phosphate and sodium after relief of UUO, and (2) the reduced basal FEP and the blunted phosphaturic response to ECVE and infusion of PTE observed in the obstructed kidney may be due to the renal hypoperfusion secondary to UUO. The data are consistent with the notion that unilateral ureteral obstruction is associated with enhanced phosphate reabsorption in the proximal tubule and decreased sodium reabsorption at a site distal to the proximal tubule.