It has been known since the 1920s that magnesium is influential in calcium homeostasis. In the 1970s it was documented that parathyroid secretion and activity may be impaired in magnesium deficiency. In the past two decades a variety of studies have indicated alterations in circulating vitamin D metabolites in humans, although these observations are not entirely consistent. Animal studies have not consistently demonstrated impaired vitamin D metabolism during relatively brief periods of magnesium deprivation, despite in vitro magnesium dependence of 1 alpha-hydroxylase activity. Studies of the administration of active vitamin D metabolites to humans and animals suggest that skeletal resistance to these compounds in magnesium deficiency may, in part, explain their reduced calcaemic effect during magnesium deficiency.