Arginine-vasopressin (AVP) is critical for the expression of a variety of social behaviors in many species. Previous studies have demonstrated that AVP regulates behaviors such as social communication and aggression in Syrian hamsters through the V1a receptor subtype. In male hamsters, AVP injected into the anterior hypothalamus (AH) stimulates aggression, while injection of a V1a receptor antagonist inhibits the behavior. The purpose of the present studies was to determine whether AVP influences aggression by its action in the AH in female hamsters. In the first experiment, we were surprised to find that injection of the V1a receptor antagonist, Manning compound, into the AH of intact female hamsters increased aggression. The second experiment confirmed the ability of the V1a receptor antagonist to increase aggression and found that the largest effects of the antagonist occurred at intermediate concentrations of the compound. The next experiment found that injection of AVP into the AH significantly reduced the latency to attack and the duration of aggression. Finally, we examined whether the effects of AVP and the V1a receptor antagonist on aggression differed in hamsters exposed to long ‘summer-like’ photoperiods or short ‘winter-like’ photoperiods, and found that their effects on aggression were not photoperiod dependent. In summary, contrary to what is observed in males, these data suggest that AVP in the AH may play an inhibitory role on aggression in female Syrian hamsters.