Light and electron microscopic immunohistochemical techniques were used to investigate the central projections and colocalization relationships of a subpopulation of primary afferent neurons that were immunolabelled with an antibody (AB893) against rat liver gap junctions. In lumbar dorsal root ganglia AB893-immunoreactivity was seen in 14.5% of all cells and in both small and large size neurons. Colocalization analysis showed that 78% of all AB893-immunoreactive (AB893-IR) neurons contained calcitonin gene-related peptide, while only 7 to 10% contained the calcium binding proteins parvalbumin or calbindin D28k. Among small type B AB893-IR ganglion cells, it was calculated that over 90% contained fluoride-resistant acid phosphatase, while only 1 to 2% contained substance P or somatostatin. Cytochrome oxidase histochemistry revealed light staining in the vast majority of AB893-IR cells. In the dorsal horn of the spinal cord the antibody labelled fibers in the dorsal root, Lissauer's tract, lamina I and lamina II. Isolated immunoreactive fiber bundles were arranged in sheets spanning most of lamina II. Immunoreactive fibers were depleted from the dorsal horn after dorsal rhizotomy or neonatal capsaicin treatment. Ultrastructural examination showed that AB893-IR fibers were composed of closely associated clusters of 2 to 5 unmyelinated fibers each ranging from 0.1-0.4 microns in diameter. Immunoreactivity was distributed intermittently along the cytoplasmic membrane of axons and en passant sinusoid terminals located centrally within the fiber clusters, as well as along axonal membranes adjacent to the central axon or terminal. The results suggest that the immunoreactive fibers in lamina II of the dorsal horn originate from a subpopulation of AB893-IR neurons that contain FRAP and give rise to unmyelinated axons.