We tested the hypothesis that aging alters physiological regulation of the serum concentration of 1,25-dihydroxyvitamin D [1,25(OH)2D] by inorganic phosphorus. In seven elderly men [age 71 +/- 1 (SE) yr] and 9 young men (29 +/- 2 yr), dietary phosphorus was first normal, then increased and decreased within its normal range. At each intake of phosphorus, serum concentrations of 1,25(OH)2D in the elderly did not differ from those in young men, but fasting and 24-h mean serum concentrations of phosphorus were lower in elderly men. With phosphorus restriction, in each group serum 1,25(OH)2D increased by 47%, and 24-h mean serum phosphorus decreased by 0.6 +/- 0.1 mg/dl. Serum concentrations of 1,25(OH)2D varied inversely with 24-h mean serum phosphorus (R= -0.92, P<0.0001). Thus, in healthy elderly men in whom glomerular filtration rate is normal or near normal, serum concentrations of 1,25(OH)2D increase when dietary phosphorus is restricted; the magnitude of response at steady state is unaffected by aging, but the time course of response is delayed. At any level of serum phosphorus, serum 1,25(OH)2D is lower than that in young men, as reflected by a lower intercept of regression of serum 1,25(OH)2D on 24-h mean phosphorus.