Several studies suggest a role for arginine vasopressin (AVP), particularly in the lateral septum, in the regulation of intermale aggression. We used intracerebral microdialysis to monitor the local in vivo AVP release within the mediolateral septum of adult male Wistar rats bred for low (LAB) or high (HAB) anxiety-related behaviour during exposure to the resident-intruder test. LAB residents showed a significantly higher level of aggression than HAB residents, as reflected by more time spent with lateral threat, offensive upright and total aggressive behaviour as well as by more attacks and a shorter attack latency. Septal AVP release was significantly decreased in high-aggressive LAB males, while septal AVP release tended to increase in HAB males during resident-intruder test exposure. Moreover, LAB residents showed reduced neuronal activation of the lateral septum, as indicated by fewer c-Fos-positive cells, 1 h after the resident-intruder test. Pharmacological manipulation of the septal AVP system by local application of either synthetic AVP to LAB residents or the selective V1a receptor antagonist d(CH(2))(5)Tyr(Me)AVP to HAB residents did not change the level of aggression. However, application of AVP into the septum enhanced anxiety-related behaviour on the elevated plus-maze in LAB males, while local administration of the V1a receptor antagonist reduced social investigation in HAB males during the resident-intruder test. In conclusion, although AVP release patterns within the septum are dependent on the level of aggression, locally released AVP does not seem to be directly involved in the regulation of aggression, but rather modulates non-aggressive social and anxiety-related behaviours.