The objective of this study was to evaluate selection responses and heterotic effects in mouse line crosses after ten generations of selection. Four mouse lines were analyzed: G, L, W and C, selected for growth (body weight at 42 days [BW42]), tail length at 42 days [TL], litter size at birth [LS], and the control line, respectively. After 10 generations of selection the first set of crosses was created; in generation 12, backcrosses and three-way crosses were made. In the crosses the following traits were analyzed: body weight at 21, 42, 63 days, tail length at 42 days, litter size and litter mass at birth. Additive genetic effects of all lines were significant for BW (at all three measurement times) and TL. Heterosis was found for BW42 for the WxC combina- tion, whereas the CxL combination tended to have a BW42 lower than expected from the line means. The same effect was observed for the CxG cross at day 63 with the effect increasing with age. With the exception of a maternal heterotic effect in the GxL cross, there was no significant effect on reproductive traits. The results show that 10 generations of line separation with selec- tion on different traits (rather than divergent selection on a single trait) are enough to create tic differences between the lines which result in a significant amount of heterosis for some parameters.