The role of septal arginine vasopressin (AVP) in a social recognition test was investigated in both homozygous Brattleboro (HO-DI) and normal Long-Evans rats. To do this, the duration of investigation of conspecific juveniles by untreated adult males of both rat strains was measured before and after inter exposure intervals of 30 and 120 min. Additionally, a microdialysis administration technique was used to administer synthetic AVP (0.2 or 2.0 ng) or its V1 receptor antagonist d(CH2)5Tyr(Me)AVP (5.0 ng) into the mediolateral septum concomitantly with the behavioral test. Untreated HO-DI rats showed an impaired social recognition compared with untreated Long-Evans rats. A similarly impaired performance was observed after V1 receptor antagonist treatment of Long-Evans rats. Microdialysis administration of synthetic AVP, on the other hand, significantly improved social recognition in both rat strains. The data suggest that endogenous AVP in the septal brain area is critically involved in the acquisition, storage, and/or recall of olfactory cues in rats.