1. Q-strain mice selected for high (QLF) or low (QSC) body-weight at 6 weeks of age were compared with respect to their body-weight increases, gross body compositions and food intakes. 2. DNA, RNA, protein and hydroxyproline contents were measured. 3. QLF animals were larger at all stages of development but ate more food and gained more body-weight per unit food intake with an apparently improved efficiency of utilization compared with QSC mice. 4. The efficiency of deposition of dietary energy in Q-strain mice was found to be significantly lower than that of other growing mammals receiving similar energy intakes. 5. Body water, protein and fat of both strains were similar at birth and at 42 d of age but the contribution of fat to body-weight in the preweaning phase was greater for QLF while QSC accreted more fat per unit weight gain in the postweaning period. 6. An increase in cell number made a greater contribution to the growth post partum of the QLF mice, but by 42 d of age little difference between the number of cells per unit weight in the two strains was evident. 7. Despite increases in RNA concentrations at all stages of development, of QLF mice compared with QSC, measurements of body composition do not indicate any accompanying increases of protein concentration in these animals.