Four diets with graded levels of energy at 24% crude protein were fed to C 57 BL/ 6 J mice for approximately 3 years to develop pelleted non-purified diets. The nitrogen-corrected metabolizable energy (MEn) of the diets ranged from 2.86 to 3.73 kcal per g of dry matter (DM) . Fifteen males and 30 females were assigned to each diet. The mice were housed together for 1 week at7 week intervals, experiencing 5 matings. After the reproduction stage, they were allowed to complete their life span. Moribund mice after 60 weeks of age were subjected to histopathological examination. The highest energy diet showed the following results in comparison with the lowest energy diet : (1) weaning weight of pups increased by 31.6% ; (2) males showed slight obesity even during the reproduction stage, but females did not ; (3) both sexes developed remarkable obesity after 50 weeks of age with 41.2% (males) and 49.6% (females) increasing in maximum body weight ; (4) although daily feed intake decreased by approximately 18%, the MEn was slightly overconsumed ; (5) females showed higher incidence of alopecia with age ; (6) the survival rate after 50 weeks of age decreased earlier and life span was shortened ; (7) histopathological changes associated with aging developed earlier. On the other hand, the early death rate of dams at parturition increased with a decrease in dietary energy. It was concluded that at least 2 types of diets are needed throughout the life span of C 57 BL/6 J mice : a high energy diet with an MEn value of 3.73 kcal/g DM for maximum reproduction and a low energy diet with an MEn value of 2.86 kcal/g DM for the long term stage after reproduction to retard obesity and aging most effectively.