An affinity-purified antibody against a 27-kD rat liver gap-junctional protein (GJP) was used to determine the distribution of GJP immunoreactivity in sections of rat hippocampus. Four heterogeneously distributed GJP-immunostaining patterns were observed. The two most common were punctate immunoreactive elements ranging in size from 0.3 to 0.7 microns and networks of immunoreactive varicose fibers coursing in a variety of directions within the various hippocampal layers and ranging in length from a few microns up to 200 microns. The density of punctate immunostaining was highest within a portion of the stratum pyramidale, at the border between the stratum pyramidale and stratum oriens, and at the border between the molecular and granule cell layers of the dentate gyrus. Moderate to low densities were observed in other hippocampal areas. Immunoreactive fibers were most concentrated within the border portions of the stratum pyramidale and oriens, moderately distributed in the stratum radiatum and the remaining part of the stratum oriens, and sparse in the alveus. In the dentate gyrus, fiber networks were most evident at the border between the granule cell and molecular layers and very unevenly distributed in the molecular layer. The two other patterns observed included intense filamentous immunostaining within a small number of neuronal perikarya located mainly in the stratum pyramidale of areas CA2 and CA3, but rarely in area CA1 or the dentate gyrus, and diffuse immunostaining of small cell bodies dispersed throughout the hippocampus but most numerous in the vicinity of the stratum pyramidale and in the alveus. All of these immunostaining patterns were seen at all rostrocaudal hippocampal levels. These results suggest that if GJP-immunoreactive fibers and neurons observed in the hippocampus have the capacity to form gap junctions, then electrotonic transmission may constitute an important means of information processing within this structure.