【Abstract】 Objective This study aimed to examine the effects of tryptophan hydroxylase 2 (Tph2) gene knockout on social play behavior during adolescence and aggressive behavior in adulthood in male rats, to examine the role of central serotonin (5-HT) synthesis deficits in social behavior across postnatal developmental stages. Methods Male Tph2 knockout (Tph2KO) and wild-type (WT) rats were used in this study. Locomotor activity and anxiety levels during adolescence were evaluated after 4 weeks of age through the open field test. The 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 through the open field test. The adult aggressive behavior was assessed in rats at 10 and 11 weeks of age using the resident-intruder test. Results In the open field test during adolescence, Tph2KO male rats exhibited a significantly shorter total travel distance compared to WT male rats (P < 0.05), while there was no significant difference in central zone exploration time or central zone entries between the two groups (P > 0.05). In the adolescent social interaction test, Tph2KO rats displayed a significant increase in social play behavior (P < 0.05) and a reduction in neutral social behaviors (P < 0.01) compared to WT controls, while there were no significant differences in overall social behaviors (P > 0.05). In the open field test during adulthood, Tph2KO male rats exhibited significantly reduced total travel distance (P < 0.05), a trend toward decreased central zone exploration time (P = 0.1003), and significantly fewer central zone entries (P < 0.05) compared to WT rats. In the resident-intruder test, there were no significant differences in overall social behavior, aggressive behavior, or neutral social behaviors between the two genotypes either (P > 0.05). Notably, Tph2KO rats exhibited increased same-sex mounting behavior during both adolescence and adulthood (P < 0.05, P = 0.076). Conclusion Tryptophan hydroxylase 2 gene knockout significantly increased social play behavior during adolescence and elevated same-sex mounting behavior during both adolescence and adulthood in male rats, while having no significant effect on adult aggressive behavior. These findings suggest that central 5-HT synthesis deficiency exerts a selective modulatory effect on adolescent social behaviors and impacts same-sex mounting behavior across developmental stages.