Although the neurobiological mechanisms underlying autism spectrum disorder (ASD) are still unknown, dysregulation of serotonergic systems has been implicated in the etiology of ASD, and serotonergic antidepressant drugs are often prescribed to treat some symptoms of ASD. The BALB/c strain of mice express a dysregulated serotonergic system and a phenotype that is relevant to ASD. In this study, juvenile male BALB/c mice were exposed to the selective serotonin reuptake inhibitor fluoxetine either chronically (18 mg/kg/day in drinking water, post-natal day (PND) 28-39) or acutely (18 mg/kg, i.p.; PND40), or to vehicle control conditions (0.9% sterile saline, i.p.; PND40), prior to being exposed to the three-chambered sociability test (SAT; PND40). One cohort of mice then received an injection of the aromatic amino acid decarboxylase inhibitor, NSD-1015, and one hour later brain tissue was collected for quantification of 5-hydroxytryptophan accumulation in the dorsal raphe nucleus (DR) as a measure of TPH2 activity. For the second cohort, brain tissue was collected ninety minutes after the onset of the social phase of the SAT and prepared for immunohistochemical staining for c-Fos and TPH2 to measure the activation of serotonergic neurons within subregions of the DR. Acute fluoxetine decreased social behavior, while chronic fluoxetine increased social behavior compared with vehicle-treated controls. Furthermore, acute and chronic fluoxetine treatments were without effect on TPH2 activity but differentially affected populations of serotonergic neurons in the DR. These data are consistent with the hypothesis that serotonergic systems are implicated in social behavior that is relevant for ASD.