Measurements made in the open-field test of the ambulation and defecation scores of rats from a 6 $\times $ 6 diallel cross were examined for their bearing on the interaction of genetic and environmental determinants of behaviour. The analyses, which were developed especially for this purpose, relate both to developmental stability and change in response to the test situation on 4 successive days. For the first, family variances were used as the variable, and for the second both polynomial components of regression of the individual daily scores as well as heirarchical analysis of variance. Otherwise standard biometrical genetical analyses were employed. The results indicate that, while the stability of open-field defecation and ambulation is influenced during development by genetical determinants largely relating to dominance, there is little evidence of interaction with environment during this time. The analyses of response to test experience, however, show complex relationships which, moreover, differ as between the two aspects of behaviour studied. Some speculations about the evolutionary significance of these results are offered.