Protein kinase C (PKC) is one of the major cellular signal transduction systems. Since at least nine different PKC isoenzymes have been described, the purpose of the present studies was to identify the regional, cellular, and subcellular distributions of PKCδ in the rat central nervous system (CNS) by light and electron microscopic immunocytochemistry. We have found that PKCδ immunoreactivity is present in all major subdivisions of the rat CNS. Within each of the subdivisions, PKCδ immunoreactivity is localized to perikarya that monitor sensory and motor functions. More specifically, PKCδ is found in the olfactory bulb, cerebral cortex, lateral septum, thalamus, vestibular and cochlear nuclei, inferior olive, nucleus of the solitary tract, cerebellum, and superficial layers of the dorsal horn in the spinal cord. In most cases, the distribution of this isoenzyme is distinct from that of the conventional isoforms. Within the CNS, PKCδ is localized primarily in neurons; however, neurons of the same type are not uniformly labeled. This is most evident in the cerebellum, where alternating columns of Purkinje cells are immunostained. While PKCδ is prominent in perikarya, occasional immunostaining is seen in dendrites, fibers or axons, and nerve terminals. Electron microscopic analysis of the posterolateral nucleus of the thalamus reveals that the cell nucleus, the rough endoplasmic reticulum, and the plasma membrane are all immunopositive. Since each of the PKC subspecies may have different substrate, lipid, and other co‐factor requirements, the regional, cellular, and subcellular distribution of each of these isoforms should help to define their functional environments. © 1993 Wiley‐Liss, Inc.