Wildcaught cinnamon ringtail monkeys, Cebus albifrons, were fed diets with Ca:P ratios of 1:4 1:2.1 1:0,4, and 1:0.5 for 3 to 88 months. Monkeys fed the diet with Ca:P ratios of 1:4 and 1:21 C ratios similar to that of human diets) had minor microscopic changes suggestive of osteoporosis when compared to other species of animals. The changes were not detected by conventional or magnification radiography or by 125I photon absorptiometry. These findings are in in striking contrast to studies in other animals where similar diets resulted in significant bone resorption within 6 weeks to 6 months. This study suggests that the non-human primate may be a more appropriate animal model for the investigation of nutritional osteopenia in man in whom bone resorption appears to be a slowly progressive process. In view of our findings, studies using lower animal species must be re-evaluated with respect to the hypothesis that high dietary phosphate is a significant etiologic factor in senile osteoporosis in man.