We have localized the beta I-, beta II-, and gamma-subspecies of protein kinase C in cerebral neocortex with light and electron microscopic immunocytochemistry using a monoclonal antibody against gamma-PKC and polyclonal antisera to beta I- or beta II-PKC-specific oligopeptides. The gamma-PKC-immunopositive cell bodies were seen mostly in layers II, V, and VI, and the vast majority of them were pyramidal cells. The beta II-PKC immunoreactive cell bodies were observed in layers II, III, V, and VI, and most of them seemed to be pyramidal cells. Both gamma- and beta II-PKC were colocalized in some pyramidal cells in layers II, V, and VI. The small number of beta I-PKC immunoreactive cell bodies were observed in the neocortex, and many of them were nonpyramidal cells. About 80% of the beta I-PKC- immunoreactive cells were shown to be GABAergic neurons. The gamma-PKC- immunopositive neuropils were observed in layers I, II, V, and VI, while beta II-PKC-immunoreactive neuropils were seen in layers I-III, V, and VI. The distribution of each subspecies is much the same throughout all regions of the neocortex, although with different intensities of immunoreactivity. electron microscopic studies revealed that, in the perikarya, gamma-PKC was distributed throughout the cytoplasm, beta I-PKC was just adjacent to the plasma membrane, and beta II-PKC was located around the Golgi complex. The immunoreactivity of these 3 subspecies was also seen in dendrites and axons, but no immunoreactivity of these subspecies was found in the presynaptic terminals in the present study. The discrete cellular and intracellular distributions of protein kinase C subspecies imply that each subspecies has a specific role in neuronal activity in the cerebral neocortex.