This experiment examines the consequences and welfare implications of the introduction of new rats to established (resident) caged laboratory rat groups. The effects on investigatory and aggressive behaviours of the established and the newly introduced rats were measured. The aggressive status within each established resident group was known prior to the introduction; the status of each intruder rat was also known. Single-sex groups of both male and female residents and intruders were studied. Two important conclusions arise from the results of the experiment. First, contrary to reports in a number of publications, there were no effects of male resident social status on their investigatory or aggressive behaviour towards male intruder rats. There was, however, a significant effect of social status among female residents on their investigatory behaviour towards intruders; unexpectedly, it was the subordinates followed by the dominant animals that showed the highest investigatory response. Second, aggression by intruders of both sexes depended upon their social status in their resident group. In the case of male intruders, aggressive behaviour towards residents was a reflection of their previous social status irrespective of how much aggression they received from residents. Among females, a similar pattern emerged, with the dominant females initiating the most aggression; however, this behaviour was dependent upon how much aggression was received from residents and on the resident’s social status, with resident subordinates and dominants responding with the most aggressive behaviour. The results are interpreted in terms of putative competitive strategies adopted by rats in a confined (cage) environment, and their welfare implications are discussed.