Effects of dietary calcium intake on bone mineral contents, density and strength in marginally vitamin D-deficient rats investigated. Vitmin D-deficient rats were fed ad libitum one of the diets containing various calcium concentrations (0.22, 0.44, 0.88 and 1.20%) prepared by L-calcium lactate with oral supplementation of a low level of vitamin D_3 (0.35 IU/rat, twice a week) for 28 days. During and after feeding, concentrations of calcium, phosphate, 25-dhyroxyvitamin D_3, 1 α, 25-dihydroxyvitamin D_3 and alkaline phosphatase activity in plasma were measured. Furthermore, femoral ash weight, mineral density and strength were also measured. In spite of marginally vitamin D deficiency, increased calcium intake provided increased plasma calcium level, bone growth, mineral density and strength. However, the magnitudes of increased bone mineral contents, density and strength in marginally vitamin D-deficient rats were significantly lower than those in vitamin D-sufficient rats reported previously. These results suggest that sufficient amount of vitamin D supplementation with an appropriate calcium intake are essential for completing normal bone growth, although increased calcium intake increased some extents of plasma calcium level and bone mineral contents, density and strength even in marginally vitamin D deficiency.