Distributions of mu- and kappa-opioid receptor mRNAs in the lumbar spinal cord and dorsal root ganglia of the adult rat were examined using the in situ hybridization technique. In the lumbar spinal cord, mu-opioid receptor mRNA was expressed intensely in laminae I, II and VIII. On the other hand, kappa-opioid receptor mRNA was expressed intensely in laminae I and II, and moderately throughout laminae III-VIII. In the dorsal root ganglia, mu-opioid receptor mRNA was intensely expressed and kappa-opioid receptor mRNA was expressed in a smaller number of cells than mu-opioid receptor mRNA.