The arginine vasopressin (AVP) system plays an important role in social behavior. Autism, with its hallmark disturbances in social behavior, has been associated with the V1a receptor (V1aR) gene. Furthermore, impairments of social function are often observed in symptoms of schizophrenia. Subchronic phencyclidine (PCP) produces behaviors relating to certain aspects of schizophrenic symptoms such as impairing social interaction in animals and it reduces the density of V1aR binding sites in several brain regions. Here, we report that V1aR knockout (KO) mice exhibited impairment of social behavior in a social interaction test, and showed reduced anxiety-related behavior in elevated plus-maze and marble-burying behavior tests. Given the current findings, the V1aR may be involved in the regulation of social interaction, and V1aR KO mice could be used as an animal model of psychiatric disorders associated with social behavior deficits, such as autism and schizophrenia.