Technical Abstract: Caloric restriction initiated in young mice and rats results in increases in mean and median lifespan. When calorie restriction is implemented in older animals an increase in lifespan is still observed, however the magnitude of the increase is not as great as that observed in animals calorie restricted since they were young. Here we report the results of a study in which calorie restriction was initiated in mature, older rats. Survival rates and terminal pathology were characterized and compared between a cohort of 17 continually ad libitum fed Long Evans rats and a cohort of 18 Long Evans rats which were gradually introduced to 33% restriction in diet consumption at 18 months of age. No difference in the median lifespan was observed between the two groups. This data may suggest that there may be a level of maturity or a stage in the aging process after which caloric restriction no longer increases longevity.