Correlated responses in growth, body composition and efficiency were evaluated in lines of mice selected in the following ways: W(+)T i (o) , increased six-week body weight (WT6); W ยฐ T i (+) , increased six-week tail length (TL6); W(+)T i (-) , increased WT6 and decreased TL6; W(-)T i (+) , decreased WT6 and increased TL6; M16, increased three-to six-week postweaning gain (PWG). Each of the first four selection treatments had two replicate lines (i = 1, 2) selected for 13 generations and the fifth treatment had one line selected for 30 generations. All lines were derived from a randombred ICR albino population which served as a control. Additional traits studied were three-week body weight and tail length, postweaning gain in tail length, percent body composition (ash, fat, moisture and protein) at six weeks of age, and three-to six-week feed consumption (CONS) and efficiency (EFF = PWG/CONS). Efficiency of body constituent gains (ash, fat, protein and caloric value) were determined by dividing each constituent by CONS. Relative to selection treatments, replicate variation in the array of traits was small and was primarily attributable to the effects of genetic drift; more frequent significant replicate differences among traits in W(+)T(-) were associated with a replicate difference in cumulative selection differentials. Selection for different criteria involving WT6 and TL6 did not change the allometric relationship between tail length and body weight in the three-to six-week age interval. The significant divergence between W(+)T ยฐ and W ยฐT(+) and between W(+)T(-) and W(-)T(+) was as expected for WT6 and TL6. Significant asymmetry of selection response between W(+)T(-) and W(-)T(+) for WT6 and TL6 was attributed to maternal effects. In agreement with theory, antagonistic index selection generally yielded smaller genetic responses than single trait selection. Positive correlated responses in CONS and EFF were found for M16 and W(+)T ยฐ. Significant correlated changes in CONS (positive in W ยฐT(+) and negative in W(-)T(+)) were not accompanied by a significant change in EFF. In contrast, W(+)T(-) evinced an increased EFF and no change in CONS. Percent fat increased significantly in W(+)T ยฐ and M16. For W(+)T(o), W(+)T(-) and M16, an increased energetic, fat and ash efficiency was observed, whereas M16 exhibited a positive increment in protein efficiency as well. Among selection treatment means, there were high positive correlations between WT6 and fat weight, protein weight, percent fat, CONS and EFF and a high negative correlation between WT6 and percent protein.