Regression equations are described which allow the calculation of the calories of energy consumed by a carnivore fed on whole laboratory mice (Mus musculus) given the individual weights of the mice eaten. The live—weight caloric density of mice weighing 4.8 g or less may be determined by the linear relationship: kcal/g live weight = 0.657 + 0.306 (g live body weight). Mice weighing more than 4.8 g may be considered to have a uniform live—weight caloric density of 2.14 ° 0.08 (SE) kcal/g. These conversion factors may be used for either frozen or freshly killed laboratory mice or laboratory raised Peromyscus polionotus. Fat indices (g fat/g lean dry weight) were not correlated with body weight. However, water indices (g water/g lean dry weight) decreased exponentially with increasing body weight. There was no change in the caloric value of the lean dry weight of the mice with increasing body weight.