To determine whether aging alters the metabolism of 1,25-dihydroxyvitamin D [1,25-(OH)2D] in men, we measured the serum concentrations, MCRs, and production rates of 1,25-(OH)2D in healthy old (age, 72 +/- 5 yr; n = 9) and young men (age, 34 +/- 5 yr; n = 9) consuming a constant metabolic diet and in whom the glomerular filtration rate was greater than 1.2 mL/s.1.73 m-2. The results indicate that when dietary calcium and phosphorus are normal and glomerular filtration rate is not reduced, the serum concentrations, MCRs, and production rates of 1,25-(OH)2D in old men [83 +/- 22 pmol/L; 0.62 +/- 0.10 mL/s.70 kg ideal BW (IBW); 51 +/- 12 fmol/s.70 kg IBW, respectively] and young men (90 +/- 20 pmol/L; 0.56 +/- 0.09 mL/s.70 kg IBW; 52 +/- 13 fmol/s.70 kg IBW, respectively) are equivalent. Indices of serum PTH, however, were elevated in the elderly men. These results suggest that aging per se has little or no effect on the serum concentration, MCR, or production rate of 1,25-(OH)2D in men. Maintenance of a normal production rate of 1,25-(OH)2D in elderly men, however, may require increased circulating PTH. Most observed declines in serum 1,25-(OH)2D in elderly men are probably a consequence of decreased functional renal mass.