Vitamin D must be regarded as the major humoral substance involved in the regulation of calcium metabolism in all higher organisms. In the absence of vitamin D, bones fail to acquire the required mineral for structural function, resulting in the diseases rickets in the young and osteomalacia in the adult (13). Furthermore, insufficient levels of calcium and phosphorus are provided in the plasma resulting in failure of nerves and muscles to function properly giving rise to the overt convulsive state known as hypocalcemic tetany (13). In short, the supply of calcium and phosphorus to the organs of higher animals depends on the presence of vitamin D. Therefore, the essence of vitamin D function is the elevation of plasma calcium and phosphorus to levels that will support mineralization of bone and provide for adequate neuromuscular function. To carry out this basic function, vitamin D stimulates intestinal absorption of calcium and phosphorus (13). In addition, vitamin D plays a major role in the parathyroid-hormone-directed mobilization of calcium from bone. Furthermore, vitamin D may be involved in the parathyroid-hormone-dependent stimulation of calcium reabsorption in the distal renal tubule (70). These sites of action of vitamin D result in the transfer of calcium from the lumen of intestine, from the lumen of the distal renal tubule, and from the bone fluid compartment to the plasma compartment. Thus a primary function of vitamin D is to stimulate transport of calcium transcellularly across a membrane of cells. These functions are diagrammatically represented in Fig. 10. 1. Please note that vitamin D is not involved in the actual transfer of calcium from the plasma compartment to the mineralizing sites of bone (75). This has recently been quite clearly excluded as a site of action of vitamin D. For vitamin D to carry out the functions of stimulating the movements of