We cloned a cDNA for the rat kappa-opioid receptor from a rat thalamus cDNA library. The deduced amino acid sequence consists of 380 residues with features shared by members of the G protein-coupled receptor family. The specific binding of [H-3]bremazocine to the membrane of COS-7 cells transfected with the cDNA was displaced by kappa-specific opioid ligands, but not by mu- and delta-specific ligands. Xenopus oocytes injected with the in vitro transcribed mRNA responded to opioid ligands with the same subtype specificity. Northern blot analysis demonstrated that kappa-opioid receptor mRNA is expressed in a regionally specific manner in rat brain.