The purpose of this study was to determine the effect of food restriction on age-related changes in serum 1,25-dihydroxyvitamin D and PTH, two important regulators of Ca metabolism. Starting at 6 wk, male F344 rats were fed a purified diet either ad libitum (non-restricted) or 60% of ad libitum (restricted). Rats from each group were killed at 5, 13, 22 and 28 mo of age. Dietary restriction increased the median lifespan from 24 to 31 mo. It delayed the rapid decrease in serum 1,25-dihydroxyvitamin D from 1.5-5.0 mo in the non-restricted group to 5-13 mo in the restricted group. It also completely suppressed the marked rise in serum PTH which occurred at 22 and 28 mo in the non-restricted group. Dietary restriction had these effects even though both groups of animals consumed the same amount of Ca per gram body weight. Diet had no effect on serum Ca and P, except at 28 mo. These effects of dietary restriction on serum 1,25-dihydroxyvitamin D and PTH may result in altered Ca metabolism in dietary restricted F344 rats.