A high level of serotonin in blood is the most consistent neurochemical change seen in autism and in family members of those with autism. In addition, the neuropeptide oxytocin is increasingly being studied and utilized as a therapeutic treatment in individuals with autism as well as being an important component in mediating social behavior. While it is known that serotonin stimulates oxytocin release, the relationship between serotonin and oxytocin has been little studied with regard to its effects on social behavior. Some commonly used antidepressants such as Prozac, which typically act to increase serotonin availability, have been increasingly used during pregnancy over the past decades, and have been identified as a significant risk factor for autism, perhaps contributing to the increase in autism prevalence. This study will investigate the role of serotonin-oxytocin interactions in social behavior in two monogamous animal species, the titi monkey and the prairie vole. In three experiments, we will examine the effects of prenatal exposure to antidepressants and chronic pharmacological activation of inhibitory serotonin receptors on social behavior, and differences in receptor distribution in the brain by sex and social status. By studying these processes it may be possible to define a mechanism tying in the high serotonin found in autism with the mechanism of autism brain development.