We have recently shown that the cytoplasmic tail of the rat mu-opioid receptor undergoes alternative splicing giving rise to two isoforms, rMOR1 and rMOR1B. These isoforms exhibit similar pharmacological profiles, however, differ in agonist-induced desensitization of coupling to adenylate cyclase. In the present study, we have raised polyclonal antibodies that specifically detect either rMOR1 or rMOR1B and used these antisera for immunocytochemical localization of the receptor proteins in the rat central nervous system. Prominent MOR1B-like immunoreactivity was found in the external plexiform layer of the main olfactory bulb localized to a dense plexus of dendrites mostly originating from mitral cells and extending into the glomerular layer. MOR1-like immunoreactivity was restricted to the perikarya of mitral cells and to distinct juxtaglomerular cells as well as their processes. While MOR1-, DOR1- and KOR1-like immunoreactivity was absent from the external plexiform layer, high densities of opioid peptides were found in this layer suggesting that MOR1B may be a targeted receptor of these peptides. MOR1-like immunoreactivity was observed in many pain-controlling brain areas including the spinal cord dorsal horn, sensory trigeminal complex, raphe nuclei and periaqueductal gray while MOR1B-like immunoreactivity was not detectable in these regions. Taken together, we provide evidence that the mu receptor isoforms, MOR1 and MOR1B, exhibit a strikingly different distribution in that MOR1 appears to be the major isoform widely distributed throughout the central nervous system and MOR1B being predominantly localized to the olfactory bulb.