Oligodendrocytes in the rat hippocampus in situ and in organotypic slice cultures were studied by light and electron microscopic immunocytochemistry using the monoclonal Rip antibody. Our results confirm that this antibody exclusively stains oligodendrocytes, while astrocytes and neurons are not labelled. In the light microscope, immunopositive cells had the appearance of myelinating oligodendrocytes with their characteristic tubular processes. In the electron microscope, stained cells showed intimate contacts with myelin sheaths but not with the basal laminae of endothelial cells. Rip-positive oligodendrocytes were unevenly distributed in the adult rat hippocampal formation. In general, they were abundant in layers known to contain many afferent and efferent fibres. In the hippocampus proper, there was a particularly strong immunolabelling of stratum radiatum of field CA2. In the fascia dentata, the hilar region displayed a high cell density, especially in the vicinity of the granule cell layer. A similar distribution of immunopositive cells was found in young animals (15-18 days old); however, the density of labelled cells was lower, particularly in the hilus. Immunolabelled cells in slice cultures of hippocampus displayed the characteristics of myelinating oligodendrocytes. Moreover, they showed an organotypic distribution, although afferent and efferent fibre projections normally myelinated by these cells were absent under these conditions.