Why should the cause of one of the commonest disorders of the human skeleton, so-called senile or post-menopausal osteoporosis, remain such an enigma? Only now does there seem to be emerging a concept of a disorder which tallies with the daily clinical observation of every roentgenologist. Nordin (8) and Harrison, Fraser, and Mullan (3) summarized the evidence indicating that osteoporosis is an atrophic process of bone associated with a negative calcium balance. The conclusion of the British investigators was predicated on their own clinical observations with dietary analysis and short-term calcium balance studies. They expressed the need for long-term studies to confirm their results and to assess the therapeutic potentialities of dietary calcium supplements. We report here the results in rats of just such long-term experiments as they suggested.