Previous research in this laboratory demonstrated the existence of a major gene (hg), expressed as a homozygous recessive, which increases postweaning growth by 60% in C57Bl/6 mice (line Ch) compared to the same genetic stock without the major gene (line CH). The effect of the hg gene on the maintenance energy requirement and net energetic efficiency of mice between 21 and 42 d of age was examined in a comparative slaughter experiment. Regressions of changes in body energy per kilogram0.75 on metabolizable energy intake per kilogram0.75 indicated that the maintenance energy requirements averaged 164 kcal/(kg0.75 X d) and 155 kcal/(kg0.75 X d) for lines CH and Ch, respectively. Net efficiency of energy utilization was 38.5 and 49.5% for lines CH and Ch, respectively. Ad libitum-fed mice of line Ch gained 46% more total body energy per unit of metabolic body weight (kilogram0.75) than ad libitum-fed mice from line CH while their energy intake per kilogram0.75 was equal. It was estimated that the decreased maintenance energy requirement and increased net energetic efficiency accounted for approximately 25 and 75%, respectively, of the increased energy gain of ad libitum-fed mice of line Ch as compared to CH. The increased energy gain of line Ch was largely (78%) due to an increase fat energy deposition. Regressions of fat energy gain per kilogram0.75 on metabolizable energy intake per kilogram0.75 indicated that the increased fat energy gain was due to an increased efficiency of fat deposition and/or an increased proportion of metabolizable energy available for gain used for fat gain.