Brain serotonin (5-HT) is one of the key regulators of aggressive behavior and social defeat stress mechanisms. We had previously found that abolishing central 5-HT biosynthesis increases aggression, as tryptophan hydroxylase 2 (Tph2-/-) knockout rats (TPH2-KO) are more aggressive than wild-type (WT) counterparts. In this elaboration, we compared different types of intruders to expand our understanding of the aggressive phenotype in TPH2-KO rats.