We cloned a cDNA for the rat mu-opioid receptor from a rat thalamus cDNA library. The deduced amino-acid sequence of rat mu-opioid receptor consists of 398 residues with the features shared by the members of the G-protein coupled receptor family, and is 59% and 60% identical with those of rat kappa-opioid and mouse delta-opioid receptors, respectively. Northern blot analysis showed that expression of mu-opioid receptor mRNA was intensive in the thalamus, striatum, hypothalamus and pons-medulla, moderate in the hippocampus and midbrain, and slight in the cerebral cortex and cerebellum. More detailed distribution of the mRNA in the rat brain was examined using the in situ hybridization technique. Intense expression of mu-opioid receptor mRNA was observed in the internal granular and glomerular layers of the olfactory bulb, caudate putamen, nucleus accumbens, medial septum, diagonal band, medial preoptic area, several nuclei of thalamus, amygdala, interpeduncular nucleus, medial raphe nucleus, inferior colliculus, parabrachial nucleus, locus coeruleus, nucleus solitary tract and ambiguus nucleus. Furthermore, mu-opioid receptor mRNA was moderately expressed in the hippocampus, globus pallidus, ventral pallidus, arcuate hypothalamic nucleus, supramammillary nucleus, superior colliculus, periacqueductal gray, and several nuclei of lower brain stem, including raphe magnus nucleus, reticular gigantocellular nucleus and lateral paragigantocellular nucleus.