We evaluated the effect of magnesium supplementation on apparent calcium absorption, bone metabolism and dynamic bone strength in ovariectomized (OVX) rats as a model of postmenopausal women. Two groups of OVX rats were fed a 0.05% Mg diet or a 0.15% Mg diet, and one group of sham-operated rats was fed the 0.05% Mg diet for 42 d. We collected feces and urine of all rats for 3-d periods starting from d 3, 10, 17, 24, 31 and 38 of the feeding experiment for calcium and magnesium balance studies. Urine was collected for 24 h from d 41 of the feeding experiment for measuring deoxypyridinoline. After the 42 d, the rats were killed, serum prepared and femora excised. The apparent calcium absorption in the OVX rats fed 0.15% Mg was significantly lower than both other groups. Additionally, the urinary excretion of deoxypyridinoline (a bone resorption marker) and the serum parathyroid hormone level of the OVX rats fed the 0.15% Mg diet were significantly lower than in the OVX rats fed 0.05% Mg. Serum osteocalcin (a bone formation marker) in the OVX rats fed the 0.15% Mg diet was significantly higher than in the OVX rats fed 0.05% Mg. The breaking force and breaking energy of the femur in the OVX rats fed the 0.15% Mg diet were significantly higher than in the OVX rats fed the 0.05% Mg diet. These results indicate that magnesium supplementation reduces apparent calcium absorption, but promotes bone formation and prevents bone resorption in OVX rats. Moreover, our results indicate magnesium supplementation increases the dynamic strength of bone.