Studies were carried out on SPF F344 male rats to evaluate the effects of aging and life-prolonging food restriction, without malnutrition, on rat skeleton and circulating PTH. Six week old F344 rats were divided into 5 groups. Group 1 rats was fed ad lib a diet that contained 21% protein. Group 2 rats were fed 60% of the mean food intake of group 1 rats from 6 wk of age for the rest of their lives. Group 3 rats were fed 60% of the ad lib food intake until 6 mo. of age and then switched to ad lib feeding. Group 4 rats were fed ad lib until 6 mo.of age, and then switched to 60% of the ad lib food intake. Group 5 rats were fed ad lib a diet that contained only 12.6% protein so that these animals ingested the same amount of protein per day as the group 2 rats. In group 1 animals, bone length, weight, density, and Ca content increased rapidly with age and plateaued at .apprx. 12 mo. of age. There was no evidence of bone loss in these animals until .apprx. 24 mo. of age, but by 27 mo., the animals had lost appreciable amounts of bone. The circulating immunoreactive PTH levels of the animals increased with advancing age, with a marked rise at 27 mo. The age-related changes in bone and serum PTH levels of rats in groups 3 and 5 were similar to those of group 1 animals, except that a terminal increase in serum PTH did not occur in group 5 rats. In the group 2 and 4 animals which were food restricted for the longest period, bone growth and maturation was slowed down, but the animals did not experience senile bone loss or marked terminal increase in circulating PTH. The salutary effects of food restriction were not due specifically to the restriction of protein intake or to restricting food intake only during the period of rapid growth.