1 alpha,25-Dihydroxyvitamin D3[1 alpha,25(OH)2D3], an active form of vitamin D, has roles in many biological phenomena such as calcium homeostasis and bone formation, which are thought to be mediated by the 1 alpha,25(OH)2D3 receptor (VDR), a member of the nuclear hormone receptor superfamily. However, the molecular basis for the actions of 1 alpha,25(OH)2D3 in bone formation, its role during development and VDR genetic polymorphisms for predicting bone mineral density are uncertain. To investigate the functional role of VDR, we generated mice deficient in VDR by gene targeting. We report here that in VDR null mutant mice, no defects in development and growth were observed before weaning, irrespective of reduced expression of vitamin D target genes. After weaning, however, mutants failed to thrive, with appearance of alopoecia, hypocalcaemia and infertility, and bone formation was severely impaired as a typical feature of vitamin D-dependent rickets type II (refs 8, 9). Unlike humans with this disease, most of the null mutant mice died within 15 weeks after birth, and uterine hypoplasia with impaired folliculogenesis was found in female reproductive organs. These defects, such as alopoecia and uterine hypoplasia, were not observed in vitamin D-deficient animals. The findings establish a critical role for VDR in growth, bone formation and female reproduction in the post-weaning stage.