There has been intensified interest in the neuropeptides oxytocin (OT) and arginine vasopressin (AVP) in autism spectrum disorders (ASD) given their role in affiliative and social behavior in animals, positive results of treatment studies using OT, and findings that genetic polymorphisms in the AVP–OT pathway are present in individuals with ASD. Nearly all such studies in humans have focused only on males. With this preliminary study, we provide basic and novel information on the involvement of OT and AVP in autism, with an investigation of blood plasma levels of these neuropeptides in 75 preadolescent and adolescent girls and boys ages 8–18: 40 with high-functioning ASD (19 girls, 21 boys) and 35 typically developing children (16 girls, 19 boys). We related neuropeptide levels to social, language, repetitive behavior, and internalizing symptom measures in these individuals. There were significant gender effects: Girls showed higher levels of OT, while boys had significantly higher levels of AVP. There were no significant effects of diagnosis on OT or AVP. Higher OT values were associated with greater anxiety in all girls, and with better pragmatic language in all boys and girls. AVP levels were positively associated with restricted and repetitive behaviors in girls with ASD but negatively (nonsignificantly) associated with these behaviors in boys with ASD. Our results challenge the prevailing view that plasma OT levels are lower in individuals with ASD, and suggest that there are distinct and sexually dimorphic mechanisms of action for OT and AVP underlying anxiety and repetitive behaviors. Autism Res 2013, 6: 91–102. © 2013 International Society for Autism Research, Wiley Periodicals, Inc.