By the use of light microscopic immunohistochemistry it was shown that a significant population of small primary sensory afferents of guinea pig contains immunoreactivities to antisera directed against prodynorphin-opioid peptides, whereas immunoreactivities to antisera directed against opioid peptides exclusively contained in proenkephalin were absent. Immunoreactivities to antisera against different prodynorphin-opioid peptides were seen in small ganglionic cells and in small diameter fibers of spinal and trigeminal ganglia, of dorsal roots, of somatic peripheral nerve trunks and in cutaneous sensory nerves. There was evidence for colocalization of different prodynorphin-opioid peptides. High-performance liquid chromatography and the mouse vas deferens assay revealed opioid-active material (22.7 pmol Met-enkephalin equivalents per gram wet weight) in extracts of somatic peripheral nerves. The results indicate for the first time that opioid peptides derived from prodynorphin are peripheral and central neurotransmitter candidates of small primary somatosensory afferents.