The maintenance of a uniform level of calcium in the serum was called by Cannon1an example of physiologic homeostasis. The regulation depends on a combination of factors, chief of which are function of the parathyroid, calcium intake, utilization and excretion, and the availability of a mobilizable calcium reserve. In considering the subject as it applies to pregnancy it becomes apparent that each of these factors assumes even greater importance than ordinarily; moreover, other relations enter into the picture to make it even more intricate. A significant fact is that the calcium level tends to decline during the later months of pregnancy. The conception that this is related to the withdrawal of calcium for fetal development is only partly tenable; it does not explain, for example, why low values occur in women with a good nutritional history whose mineral intake during pregnancy is more than sufficient to meet both