In four experiments, the effect of septal ablation on the social behavior of the hamster was observed. The results indicated that septal ablation increased the aggressiveness of both socially naive and experienced dominant hamsters. If, however, the septal hamster experienced defeat by an opponent during preoperative testing, no postoperative increment in aggression was observed. All septal hamsters, whether aggressive or not, either ceased to display hoarding behavior or hoarded very little. All septal hamsters also showed persistent approach responses to their opponents. Paired contests between septal animals resulted in high levels of aggression with one septal in each pair being defeated and assuming a submissive status.