Non-pyramidal cells in the rat hippocampus were examined with a combined Golgi-electron microscopic method. The somata of non-pyramidal cells were ovoid, about 15 X 30 micron, and several smooth and/or varicose dendrites extended from them. With electron microscopy, Golgi-impregnated gold-toned non-pyramidal cells showed distinctive fine structural features. The somata displayed large nuclei and an extensive perikaryal cytoplasm. The nuclei showed extensive cytoplasmic invaginations, little heterochromatin, conspicuous nucleoli, and intranuclear rods composed of filamentous bundles. The perikaryal cytoplasm was rich in cell organelles such as well-developed cisternae of rough endoplasmic reticulum and Golgi apparatus, and numerous clusters of free ribosomes and mitochondria. Many synaptic boutons, most of which formed asymmetrical synapses, impinged upon the somata and dendrites. Gap junctions were seen on varicose dendrites of Golgi-impregnated non-pyramidal cells. These gap junctions were patch-like, about 0.1-0.6 micron in diameter, and situated in the stratum radiatum or stratum oriens of the CA1 and CA3 regions 70-230 micron from the soma. They displayed a characteristic cytoplasmic semidense material undercoating the junctional membranes. The gap junctions were usually formed between impregnated and unimpregnated varicose dendrites. Thirteen of a total of 22 gap junctions involving the impregnated dendrites were situated singly, whereas the remaining nine were on four impregnated dendrites in clusters of two or three side by side. In the latter cases, two pairs of junctions were formed between pairs of dendrites running parallel to each other, and each of the other two pairs was formed among three dendrites, appearing to make a dendritic network bridged by gap junctions. One gap junction was seen between two impregnated dendrites originating from two identified Golgi-impregnated non-pyramidal cells. These observations revealed unequivocally that non-pyramidal cells in the hippocampus form gap junctions with one another on their dendrites.