Gap junctions were identified for the first time on chemically defined neurons in the central nervous system. Gap junctions were thus demonstrated on GABAergic neurons containing the calcium-binding on GABAergic neurons containing the calcium-binding protein parvalbumin (PV) in the rat hippocampus. Thin and semithin (0.5 micron thick) sections were cut alternately and consecutively from osmium-fixed tissue which was embedded in epoxy resin and usable for conventional electron microscopic studies. The semithin sections were processed for postembedding immunocytochemistry using an anti-PV serum. Structures corresponding to the PV-immunoreactive (PV-I) profiles on the semithin sections were easily identified on electron micrographs from the adjacent thin sections. Using this technique gap junctions were found (1) between PV-I dendrites, (2) between PV-I dendrites and PV-I somata and (3) between PV-I dendrites and small processes whose origin could not be identified. Despite a systematic search, we did not find gap junction between PV-negative processes.