Principal component analysis of behavioural measures together with body and brain weight of hybrid F2 mice crosses between two lines selected for large (LB) and small (SB) brain weight yielded eight-factor solution explaining 75.1% of total variance. Two of eight factors had sufficient loading on brain weight and several behavioural measures. The factor analysis showed that, among F2 hybrids, mice with larger brain weight were characterised, in open-field test, by higher scores of locomotion in the periphery of arena and of rearing, as well as less frequent grooming and freezing than mice with smaller brain weight. F2 hybrids with larger brain weight moved faster and displayed stereotyped behaviour in the cross-maze test more frequently. In general, this diversity is in accord with the behaviour differences between parent LB and SB lines. The results show that, in mice fear-anxiety and stereotypic behaviours, which are known to interfere with normal exploration and learning of the environment, are causally connected with brain weight.