EVER since the custom was inaugurated so brilliantly by Ashley Weech, the President of this society has been given the privilege of opening the meeting with a summary of some phase of his own research interests and since his paper is not open to discussion from the floor he can theorize a bit without the restraint of possible quick rebuttal. I have chosen to discuss the work on the "Physiology of Vitamin D" in which my wife and I have been engaged for a number of years. Since we have been in Baltimore we have been fortunate in having the benefit of Dr. Park's ideas and assistance, and the inspiration of his active interest in these studies. Research activity in this field abruptly diminished when the practical applications of the discovery of vitamin D resulted in the virtual disappearance of rickets as a clinical problem. Many of the physiological problems remain unsolved, however, and our interest has been focused on the mechanism of action of the various sterols with vitamin D activity. In the following discussion many important contributions to this problem are omitted because of limitation of time. We might first clarify certain quantitative aspects of vitamin D dosage. Figure 1 shows the Wide range of dosage over which vitamin D has been used in patients. A dose of 10 micrograms per day is adequate for the prevention of rickets in the normal infant whereas one to five milligrams per day or more have been required in the maintenance of patients with refractory rickets and hypoparathyroidism. It would seem possible that different biochemical systems might be involved at these extremes of dosage.