Within 24 hr after unilateral nephrectomy, fractional excretion of phosphate (FEp) by the remaining kidney is markedly increased. This increase in FEp occurs in throparathyroidectomized rats receiving fixed replacement doses of parathyroid hormone and, therefore, cannot be due to secondary hyperparathyroidism occurring in response to unilateral nephrectomy. In the avsence of any hormone replacement, the increase in FEp is much reduced, but still present. The increase in FEp cannot be ascribed to depression of overall tubular sodium reabsorption because it could be demonstrated in the absence of an increase in FENa. Phosphaturia following unilateral nephrectomy in the rat appears to be part of the complex of events that occur after acute reduction of renal mass; the exact mechanisms of its genesis are uncertain.