I. Introduction VITAMIN D plays an essential role in vertebrates. In concert with the two peptide hormones PTH and calcitonin, vitamin D mediates calcium and phosphorus metabolism at target tissues, including intestine, bone, and kidney. The sterol must be metabolically activated before exerting its action. Vitamin D derived from the diet or generated in the skin by photometabolism of 7-dehydrocholesterol or ergosterol undergoes 25-hydroxylation in the liver. In the kidney, 25-hydroxyvitamin D (25OHD) is further metabolized to lα,25-dihydroxyvitamin D [1,25-(OH)2D] and 24,25-dihydroxyvitamin D [24,25-(OH2)D]. The most biologically active form of vitamin D is 1,25-(OH)2D. 1,25-(OH)2D functions in a classical endocrine fashion. Its production in the kidney is under a complex regulatory system so that more of the metabolite is available when demand for calcium is high. Moreover, there is substantial evidence which indicates that, analogous to classic steroid hormones, 1,25-(OH)2D elicits its biological response in target tissues through a receptor-mediated genomic mechanism of action. The intestinal calcium binding protein is the best characterized gene product made in response to 1,25-(OH)2D. In addition, 1,25-(OH)2D may also affect intestinal calcium transport through a nongenomic mechanism (1–3).