The effects of dietary energy density on the performance of growing, gestating and lactating C57BL/6J mice were determined in order to develop pelleted nonpurified practical diets for use in all stages of the mouse life cycle. Experimental diets with 4 levels of energy at 24% crude protein (CP) were pelleted and the nutritional values were determined using adult rats. The nitrogen-corrected metabolizable energy (MEn) values ranged from 2.86 to 3.73 kcal/g and the digestive CP (DCP) contents ranged from 20.5 to 22.6% on a dry matter (DM) basis. Mice responded to decreased dietary energy by increasing their feed intake to maintain MEn intake levels, except for 1 week after weaning and during lactation periods. During these periods, mice fed lower energy diets could not consume as much MEn as those fed higher energy diets. The lowest energy diet, in comparison with the highest energy diet, resulted in approximately a 33% lower weaning weight of pups at 3 weeks of age, a 13.2 to 34.4% slower growth at 3 to 4 weeks of age, and a 9.3 day delay in the onset of vaginal opening in young females. Lower energy diets, however, did not affect the litter size or the birth weight of pups. The DCP intake usually increased with decreases in dietary energy but apparently this did not affect the performance of the mice. It was concluded that an optimal diet should have an MEn value of 3.73 kcal/g DM or more for both the one week post weaning growth period and during lactation, but a diet with an MEn value of 2.86 kcal/g DM was sufficient for growth after 4 weeks of age and during gestation.