Background: Arginine vasopressin (AVP) has a complex but crucial role in social behavior. In nonhuman mammals it facilitates social recognition and bonding while also promoting defensive, aggressive, and territorial behaviors. There has been little research in humans exploring its effect on social cognition, including the encoding of social memories. Methods: In a double-blind, randomized, placebo-controlled, between-subject design, we administered AVP (20 IU) or a placebo intranasally to 48 healthy human male volunteers and then presented 54 happy, angry, or neutral human faces. Participants returned the following day to make "remember", "know", or "new" judgments for a mix of 108 new and previously seen faces. Results: Participants who were administered AVP were more likely to make know judgments for previously seen happy and angry faces in comparison with neutral human faces. Arginine vasopressin did not influence judgments for faces that had not been presented previously. Conclusions: Administration of AVP to male humans enhances the encoding of both happy and angry social information to make this more memorable. Results suggest that AVP could facilitate both bonding and aggressive related behaviors in humans by enhancing the encoding of positive and negative social cues within everyday interactions.