Protein kinase C (PKC) has attracted wide attention as a key enzyme for the expression of long-term potentiation in the hippocampus, a basic model for memory. It is of interest to study the detailed localization of PKC subspecies in the monkey hippocampus. We used immunocytochemistry to examine the localization of PKC subspecies in the hippocampus of the monkey, Macaca mulatta. Subspecies of PKC in the monkey could be separated by hydroxyapatite chromatography and the elution profile proved to be similar to that of the rat. Antibodies against each alpha, beta II and gamma-subspecies of the rat specifically reacted with the respective subspecies of monkey PKC. The alpha-, beta II- and gamma-subspecies were distinctly distributed in the hippocampus. The beta I-subspecies was not evident in the hippocampus. While both the alpha- and gamma-subspecies immunoreactive pyramidal cells were distributed throughout the hippocampus (CA1-CA3), the beta II-subspecies immunoreactive cells were scattered only in the CA1 region. The gamma-subspecies was found in granule cells and dendrites in the dentate gyrus, in mossy fibers and in their terminals in the CA3 region. The alpha-subspecies was also present in granule cells and in the dendrites but not in the mossy fibers. Glial cells did not stain with any of the antibodies used. Electron microscopy clearly showed that the gamma-subspecies was localized in both presynaptic terminals and post-synaptic dendrites. These observations suggest that subspecies of PKC in the monkey hippocampus may be involved in distinct functions and that the gamma-subspecies of PKC may act pre- and post-synaptically in pyramidal cells of the hippocampus.