Social and sensory characteristics of post-weaning rearing environments interact to affect the behaviour of rats in a multiple Y maze, on measures of ambulation, emergence latency and weight. On measures of running time, variability, and activity box scores the two variables act independently. Sex is seen to interact with one or both of these variables on all measures. It is concluded that greater clarification is needed in generalisations concerning types of rearing environments, and that the sex of the animals and the testing situation utilized should be considered in such generalisations. Some theoretical interpretations are discussed and suggestions for further research put forward.