Calcium absorption was measured by 47Ca disappearance from ligated duodenal loops and 47Ca uptake by tibiae in chicks fed diets containing one of four levels of Mg for two weeks. The diets, otherwise identical, contained 88 (group 1), 150 (group 2), 200 (group 3) and 1000 (group 4) ppm Mg. There were no statistically significant differences in 47Ca absorption. Mucosal CaBP and intestinal alkaline phosphatase were slightly, but not significantly, elevated in the Mg depleted chicks in groups 1 and 2 compared to groups 3 and 4. Microscopic examination of the tibiae showed marked alterations in morphology in chicks fed the 88 ppm Mg diet. The thickness of the epiphyseal plate was reduced, and the morphologic signs of osteolysis and chondrolysis, normally observed in the metaphysis, appeared to be absent. The mid-diaphysis was thickened and showed marked reduction in both osteoblast and osteocytie activity. Blood calcium levels were significantly reduced in the Mg deficient chicks. It was concluded that Mg depletion in chicks altered Ca homeostasis primarily by changing bone structure and function.