Nociceptin has been suggested to be involved in nociceptive processing in the spinal dorsal horn. To investigate whether the biosynthesis of nociceptin is altered in animals with nociceptive hypersensitivity, we examined effects of peripheral inflammation on the expression of prepronociceptin mRNA, especially in the dorsal root ganglion of the rat. Although little or no expression of prepronociceptin mRNA was observed in the dorsal root ganglia of naive animals, carrageenan-elicited inflammation markedly induced its expression, peaking within 30 min, a time when nociceptive hypersensitivity was not yet apparent. The results suggest that peripheral inflammation induced nociceptin expression in primary sensory neurons, which may be associated with the production of nociceptive hypersensitivity.