The energetics of captive house mice fed two separate foods, sheep nuts and dog biscuits, were studied and compared. Short term experiments on food consumption, assimilation, and metabolism were made and the results combined in the form of energy budgets for representative 12 and 25 g mice. Growth of three litters fed the experimental foods was also determined. Mice fed sheep nuts consumed more, assimilated a higher percentage of the food and grew to a greater size. As growth rates of young mice fed the two foods were similar it seems that sheepnut-fed mice were using more energy in respiration, probably due to higher levels of activity.