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. The resident-intruder paradigm was applied to evaluate the territorial aggression of rats, where isolated residents (TPH2-KO, WT male, 36 weeks, n=9), were presented with socialized intruders, either 50% smaller (S-I) or 30% larger by weight (L-I).