A partial cDNA encoding a κ-opioid receptor was isolated and used to generate specific 35 S-labeled probes to investigate the gene expression of the κ-opioid receptor in sensory, sympathetic and spinal neurons of the rat by in situ hybridization. A subpopulation of mainly small and medium-sized neurons within dorsal root and trigeminal ganglia expressed κ-receptor mRNA, but no signal was detectable in the superior cervical ganglion. κ-Receptor mRNA was distributed over neurons throughout the dorsal horn and in laminae VII VIII. Highest concentrations of positive neurons were seen in laminae I II, dorsal lamina X and in the lateral spinal nucleus. α-Montoneurons and glial cells were not labeled. This distribution of κ-receptor mRNA indicates preferential functions of κ-receptors in sensory signalling with particular importance to nociception.