To determine whether aging impairs renal responsiveness to PTH, we studied the ability of the kidney to respond to infusion of human PTH-(1-34) for 24 h in 16 healthy men, 9 young (39 +/- 1 yr) and 7 elderly (70 +/- 1 yr), free of any conditions known to influence mineral metabolism and in whom the glomerular filtration rate was greater than 1.17 mL/s.1.73 m2. Basal concentrations of blood ionized calcium, serum 1,25-dihydroxyvitamin D [1,25-(OH)2D], and urinary calcium and phosphorus were similar in both age groups, but basal serum PTH (+148%), plasma cAMP (+44%), nephrogenous cAMP (+56%), and fractional excretion of phosphorus (+44%) were higher in the elderly. PTH infusion increased serum 1,25-(OH)2D to the same maximum level in the young (113 +/- 10 pmol/L) and the elderly (106 +/- 11 pmol/L) men, but the response in the elderly was delayed. Urinary cAMP, nephrogenous cAMP, and fractional excretion of phosphorus increased, and neither the time course nor the increment was significantly different between age groups. The phosphate threshold concentration decreased in response to PTH to the same extent in both age groups. The results demonstrate that although the increment in serum 1,25-(OH)2D in the elderly may be delayed relative to that in the young, renal responsiveness to a 24-h PTH infusion is similar in healthy young and elderly men in whom glomerular filtration rate is normal or near normal.