The vasopressin 1b receptor (Avpr1b) is one of two principal receptors mediating the behavioral effects of vasopressin (Avp) in the brain. Avpr1b has recently been shown to strongly influence social forms of aggression in mice and hamsters. This receptor appears to play a role in social recognition and motivation as well as in regulating the hypothalamic-pituitary-adrenal axis. Most of these studies have been performed in knockout mice, a species in which the localization of the Avpr1b has not been described, thus precluding correlations with the behaviors. We performed in situ hybridization histochemistry (ISHH) with specific probes and found especially prominent expression within the CA2 pyramidal neurons of the hippocampus, with much lower expression in the hypothalamic paraventricular nucleus and amygdala. Reverse transcriptase-polymerase chain reaction (RT-PCR) confirmed expression in those as well other areas in which the ISHH was not sensitive enough to detect labeled cells (e.g. piriform cortex, septum, caudate-putamen and lower brainstem areas). Mouse Avpr1b transcript levels were not altered in the CA2 field by restraint stress or adrenalectomy. Finally, ISHH and RT-PCR showed expression of the Avpr1b gene in the rat and human hippocampi as well. We suggest that the CA2 field may form or retrieve associations (memories) between olfactory cues and social encounters.