Objective To examine the effects of tryptophan hydroxylase 2 (Tph2) gene knockout on social play behavior during adolescence and on aggressive behavior in adult male rats, in order to elucidate the role of central 5-hydroxytrypatamine (5-HT) synthesis deficits in social behavior during postnatal development. Methods Male Tph2-knockout (Tph2^KO) and wild-type (WT) rats were used in this study. Locomotor activity and anxiety levels during adolescence were evaluated after 4 weeks of age using an open-field test and adolescent social play behavior was assessed at 5 and 6 weeks of age using the adolescent social-interaction test. Locomotor activity and anxiety levels during adulthood were evaluated at 8 weeks of age by open-field test, and adult aggressive behavior was assessed at 10 and 11 weeks of age using the resident-intruder test. Results Adolescent Tph2^KO male rats exhibited a significantly shorter total travel distance in the open-field test compared with WT male rats (P<0.05), but there was no significant difference in central zone exploration time or central zone entries between the two groups (P>0.05). Adolescent Tph2^KO rats also displayed significantly increased social play behavior (P<0.05) and reduced neutral social behaviors (P<0.01) in the social-interaction test compared with WT controls, but there was no significant difference in overall social behaviors (P>0.05). Adult Tph2^KO male rats exhibited a significantly reduced total travel distance (P<0.05), a trend towards decreased central zone exploration time (p=0.100), and significantly fewer central zone entries (P<0.05) in the open-field test compared with WT rats. There were no significant differences in overall social behavior, aggressive behavior, or neutral social behaviors in the resident-intruder test between the two genotypes (P>0.05). Notably, Tph2^KO rats exhibited increased same-sex mounting behavior during both adolescence and adulthood (P<0.05, p=0.076, respectively). Conclusions Tph2 gene knockout significantly increased social play behavior during adolescence and elevated same-sex mounting behavior during adolescence and adulthood in male rats, but had no significant effect on adult aggressive behavior. These findings suggest that central 5-HT synthesis deficiency selectively modulates adolescent social behavior and impacts same-sex mounting behavior across developmental stages.