Renal 25-hydroxyvitamin D-3 1 alpha-hydroxylase [1 alpha(OH)ase] catalyzes metabolic activation of 25-hydroxyvitamin D-3 into 1 alpha,25-dihydroxyvitamin D-3 [1 alpha,25(OH)(2)D-3], an active form of vitamin D, and is inhibited by 1 alpha,25(OH)(2)D-3. 1 alpha(OH)ase, which was cloned from the kidney of mice lacking the vitamin D receptor (VDR-/- mice), is a member of the P450 family of enzymes (P450(VD1 alpha)). Expression of 1 alpha(OH)ase was suppressed by 1 alpha,25(OH)(2)D-3 in VDR+/+ and VDR+/- mice but not in VDR-/- mice. These results indicate that the negative feedback regulation of active vitamin D synthesis is mediated by 1 alpha(OH)ase through liganded VDR.