The vitamin D endocrine system, besides playing pivotal roles in calcium homeostasis & bone mineral metabolism, is now recognized to subserve a wide range of fundamental biological functions in cell differentiation, inhibition of cell growth as well as immuno modulation. Vitamin D is a prohormone which is converted into its active hormonal form 1, 25 (OH) D^sub 2^D, 1, 25 (OH) D^sub 2^D activates its cellular receptor (VDR) which activate target genes to engender its biological actions. This review provides a summary of recent understanding of the complex actions of the vitamin D hormone 1, 25 (OH)^sub 2^ D which is a final product of 1α hydroxylation in the proximal tubular cells ...