Summary1. The purpose of this review article is to re‐evaluate and integrate many of the observations related to the physiological effects of vitamin D, using as a working hypothesis the concept that the vitamin may be acting analogously to a steroid hormone in terms of its ability to interact with genetic information and ultimately elicit a physiological response. Prior to this time the problem of the mechanism of action of vitamin D has primarily been approached from the point of view that the vitamin was acting as a cofactor for some specific enzymic reaction.2. The physiological activities of vitamin D are integrated with those of parathyroid hormone to provide a homeostatic control for the regulation of primarily calcium and secondarily phosphate metabolism. It is proposed that the role of vitamin D in this homeostatic control mechanism is older and more fundamental than that of parathyroid hormone. The interaction of vitamin D on skeletal calcium metabolism may have evolved before the effects of the vitamin on intestinal calcium absorption.3. There are several physiological defects of calcium metabolism—rickets, osteo‐malacia, vitamin D‐resistant rickets and idiopathic hypercalcaemia—all of which may be a consequence of an aberration in one or another of the interlocking steps of the vitamin D‐dependent and calcium‐dependent homeostatic control mechanism.4. The most thoroughly established action of vitamin Din vivois to promote or facilitate the intestinal absorption of calcium. Although the exact biochemical details of this process are not available, this may involve vitamin D‐mediated synthesis of the appropriate enzyme systems or the alteration of membrane structure necessary for calcium absorption. It is not yet unequivocally established whether calcium absorption is an energy‐dependent active transport process or is a passive carrier‐mediated or simple diffusion process.5. The exact action of vitamin D on bone metabolism is not as well established, but the primary effect of the vitamin is likely to mediate bone resorption. The vitamin D‐dependent activities of the cell in both the intestine and bone are to absorb calcium and transfer it to the blood.6. No direct effects of vitamin D on intestinal absorption of phosphate have been found. Furthermore the validity of a vitamin D‐mediated renal reabsorption of phosphate is questioned, for the major effects of vitamin D are cation oriented. If the renal effects of vitamin D are true, it is postulated that the mechanism of action of the vitamin here on the anion, phosphate, is fundamentally different from its cation oriented mechanism.7. There is a lag in the action of vitamin D on the vitamin mediated: (a) transport of calcium bothin vivoin rats and chicks, andin vitrowith everted intestinal slices;(b) the apparent increased permeability of intestinal mucosa; (c) increased levels of citric acid in serum or bone; (d) the increased incorporation of radioactive inorganic phosphorus into intestinal mucosa phospholipids. As shown by the use of radioactive vitamin D, this lag is not due to a lack of the vitamin in the target organs.8. Whereas large, unphysiological doses of radioactive vitamin D localize in all tissues and all subcellular fractions, small physiological doses of radioactive vitamin D localize predominantly in the nucleus of the intestinal mucosa. The amount of vitamin D localized in the nucleus would appear to be too low for the vitamin to function as a cofactor, and is more indicative of an interaction on or with deoxy‐ribonucleic acid.9. Actinomycin D, an inhibitor of DNA‐directed RNA synthesis, inhibits the action of vitamin D in mediating intestinal calcium absorption and bone resorption. Vitamin D also stimulates messenger‐RNA synthesis in intestinal mucosa within 1/2 hr. of vitamin treatment. Vitamin D may play a crucial role, along with parathyroid hormone and calcium, in a DNA, gene‐dependent, homeostatic control mechanism for cal, ium metabolism. In this system the vitamin D molecule has certain very specific structural requirements which are probably a reflection of the specificity of its receptor molecule, rather than structural requirements for