It has been repeatedly shown that human growth hormone (HGH) affects phosphorus metabolism in patients with hypopituitarism and in normal subjects. Injection of HGH leads to a decrease of urinary phosphorus within a day (1-3). Whereas serum phosphorus does not change or rises only slightly in short-term experiments (1), it has been reported that it rises with long-term treatment (4). Whereas the decrease of urinary phosphorus associated with an unchanged or rising serum phosphorus suggests that growth hormone decreases renal clearance of phosphate, the mechanism for these effects has not been defined. In the dog, growth hormone has been shown to raise glomerular filtration rate (GFR), renal plasma flow (RPF), tubular maximal secretion of para-aminohippurate (TmPAH) (5-7), and tubular maximal reabsorption of sulfate (Tmso4) (8). Effects on renal phosphate clearance have not been reported. The present study was undertaken to determine whether HGH affects the ability of the human kidney to reabsorb phosphate. For this purpose, maximum tubular phosphate reabsorption (Tmpo4) was measured before and after administration of growth hormone. Effects of the hormone on GFR and on RPF were also studied. The results indicate that HGH increases the phosphate Tm, the GFR, and the RPF.