Sprague-Dawley rats were kept at 28 degrees C from 21 to 517 days age and fed one of the two following diets: a semi-purified diet containing 502 p.p.m. of Mg (control) or the same diet containing only 120 p.p.m. (mg/kg) (low-Mg). The chronic suboptimal intake of Mg by rats fed the low-Mg diet did not result in overt signs of Mg deficiency even when Mg levels were greatly reduced in carcass, plasma, and tibia, but it significantly decreased bone strength. It is suggested that Mg deficiency in man could be a factor in the weakening of bone, commonly observed in old age, even when there are no visible signs of Mg deficiency. Studies of the human situation would be of interest.