Vitamin D receptor (VDR) null mutant mice provide a model to investigate the possible effect of vitamin D on female reproduction. Infertility in these mice has been reported but it is uncertain whether the infertility results from a lack of VDR or from the hypocalcemia that results from a lack of VDR. VDR null mutant mice and wild-type controls were fed a nonpurified, high calcium or medium calcium diet, plus a diet containing lactose and their reproductive efficiency was examined. VDR null mutant mice fed a nonpurified diet were hypocalcemic and were found to be largely infertile with 14% fertility, while the fertility percentage of normocalcemic VDR null mutant mice and wild-type mice was between 86% and 100%. A high calcium or medium calcium diet maintained 100% fertility in the VDR knockout mice; removal of the lactose from this diet did not diminish reproductive capability. Reproductive capacity of VDR null mutant mice was analyzed when they were fed purified diets containing 0.02-2% calcium. Mutant mice fed a low calcium diet (0.47%) had a lower reproductive efficiency than VDR null mutant mice fed a diet that resulted in normal serum calcium concentrations. Thus, high dietary calcium levels are required for normal reproduction in VDR null mutant female mice. It seems that the defect in reproduction reported previously for VDR null mutant mice is not the lack of a direct effect of 1,25-dihydroxycholecalciferol on reproductive function but is the result of hypocalcemia.