Abstract 1α,25-dihydroxyvitamin D 3 (1,25D 3) inhibits growth and induces apoptosis in breast cancer cells in vivo and in vitro. To examine the role of the Vitamin D receptor (VDR) in mediating the actions of 1,25D 3 at nanomolar and micromolar concentrations, mammary epithelial tumor cell lines generated in wild type (WT) and VDR knockout (VDRKO) mice were utilized. WT cells express VDR and are growth inhibited by 1,25D 3 and synthetic analogs EB1089 and CB1093 at 1 nM concentrations, while VDRKO cells do not express VDR and are insensitive to Vitamin D compounds at concentrations up to 100 nM. In the current studies, we have confirmed and extended these previous observations. At nanomolar concentrations of 1,25D 3 and all analogs tested, including EB1089, CB1093, MC1288, and KH1230, WT cells are growth inhibited and exhibit apoptotic morphology, while VDRKO cells show no growth inhibition or apoptosis. At concentrations of 1–10μM, however, 1,25D 3 and synthetic analogs induce growth inhibition and apoptotic morphology in both WT and VDRKO cell lines. These data indicate that nanomolar concentrations of 1,25D 3 and analogs mediate growth regulatory effects via mechanisms requiring the nuclear VDR, but that micromolar concentrations of Vitamin D compounds can exert non VDR-mediated effects.