The neuropeptide vasopressin is now known to be released in the brain where it plays important roles in social behaviours. We have shown that the rat olfactory bulb (OB)1 and the anterior olfactory nucleus (AON)2 contain large populations of interneurones which express and release vasopressin. In the OB, single cell recordings from mitral cells in vivo showed that vasopressin modulates the processing of information by olfactory bulb neurones. Blocking the actions of vasopressin in the OB (using antagonists or small interference RNA against the vasopressin V1a receptor, or local selective destruction of vasopressin cells with diphtheria toxin in transgenic rats) impairs the social recognition abilities of rats. The treatments impaired habituation/dishabituation to juvenile cues, but not to volatile odours or object recognition, and did not affect locomotor activity or anxiety-related behaviours. Adult rats exposed to a conspecific juvenile showed increased Egr-1 expression in vasopressin neurones in multiple subdivisions of AON as compared to animals exposed to no odour or a non-social odour. These data suggest that vasopressin neurones in the AON may also play an important role in the coding of social odour information. The findings indicate that the vasopressin process olfactory signals relevant to social discrimination and dendritic vasopressin release may be involved in filtering out familiar signals. Because this filtering is important for social recognition, it seems that the vasopressin release must depend on previous olfactory experience. In the hypothalamus, activity-dependent dendrictic vasopressin release can be conditionally regulated (primed) by recent experience; we are currently investigating whether such a mechanism in the olfactory bulb mediates conditional changes in the olfactory recognition.