The comparative distributions of the vasopressin V1b receptor (V1bR) and the oxytocin receptor (OTR) messenger RNAs (mRNAs) are described in male rat brain using in situ hybridization histochemistry. V1bR transcripts were present in forebrain and hypothalamus and were less abundant in mid- and hindbrain regions, similar to the gradient observed with OTR transcripts. Microscopic analyses indicated that V1bR expressing cells typically demonstrated the morphology of neurons and confirmed V1bR gene expression in regions including the olfactory bulb, supraoptic, suprachiasmatic, and dorsomedial hypothalamic nuclei, piriform and entorhinal cortices, hippocampus, substantia nigra, and dorsal motor nucleus of the vagus. Most regions that expressed V1bR mRNA also expressed OTR mRNA, although OTR gene expression was much more extensive than that of the V1bR. V1bR and OTR mRNA distributions were distinct from each other and from that of the V1a receptor mRNA in brain. A few brain regions express only V1bR transcripts such as the dorsomedial hypothalamic nucleus and the external plexiform layer of the olfactory bulb. Other brain regions, such as the fields of Ammon's horn, the suprachiasmatic nucleus, the substantia nigra pars compacta, and the piriform cortex express mRNAs that encode all three receptor subtypes (V1a, V1b, and OTR), whereas brain areas including the red nucleus and supraoptic nucleus express V1bR and OTR transcripts only. These data suggest functional specialization of the V1b, OTR and V1a receptors in brain.