The influence of estrogen treatment (estradiol benzoate, 1 mg/kg daily for 5 days) on the in vitro renal metabolism of 25-[3H]hydroxyvitamin D3 has been studied in young adult (8-wk-old) and aged (108-wk-old) female and male Japanese quail maintained either on a normal calcium diet or on a low-calcium diet for 4 wk. It is concluded that senescence leads to reduced production of 1,25-dihydroxyvitamin D2 [1,25(OH)2D3] in untreated birds of both sexes and under both dietary conditions. Nevertheless, the renal vitamin D-endocrine system in aged birds has the capacity, when stimulated under appropriate conditions (estradiol-treated, calcium-deprived aged males), for operating at a very high production rate (315 +/- 21 pmol . min-1 . g kidney-1). The latter rate is not far below the maximal rate (393 +/- 25 pmol . min-1 . g kidney-1) observed in young adult birds that had been stimulated by calcium deprivation.