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Four diets with graded levels of energy at 2496 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 at? 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 : iD weaning weight of pups increased by 31.6% ; ® males showed slight obesity even during the reproduction stage, but females did not ; ® both sexes developed remarkable obesity after 50 weeks of age with 41.2% (males) and 49.6% (females) increasing in maximum body weight ; ® although daily feed intake decreased by approximately 18%, the MEn was slightly overconsumed ; ® females showed higher incidence of alopecia with age ; ® the survival rate after 50 weeks of age decreased earlier and life span was shortened ; 00 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.KEY WORDS : aging, alopecia, diet, mice, obesityIn the previous experiment [1], the effect of the energy density of diets on the performance of growing young C 57 BL/ 6 J mice and gestating or lactating primiparous mice was studied under the condition of sufficient protein ingestion. This was accomplished via 4 pelleted non-purified diets with graded levels of energy at 24% crude protein content. The MEn value of the diets determined with rats ranged from 2.86 to 3.73 kacl/g dry matter (DM). These were considered practically the minimum and maximum. The result showed that a nitrogen-corrected metabolizable energy (MEn) of 3.73 kcal/g DM or more was necessary for maximum performance of growth during the 1 week after weaning and during lactation periods, in terms of growth rates for suckling or weanling mice and in the earlier onset of sexual maturity.For growth after 4 weeks of age and during gestation periods, however, the MEn of 2.86 kcal/g DM was sufficient in terms of the growth rate of young mice and the birth weight of newborn pups.In the present experiment, the same diets used in the previous experiment were fed to mice of the same strain up to as long as approximately 3 years, in order to study the effect of energy density of diet on repeated

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