The subcellular localization of CD38 in the rat cerebral and cerebellar cortices was studied using immunoelectron microscopy. In the cerebral cortex, immunoreactivity was present in a subset of pyramidal neurons, and was distributed predominantly in the perikarya and dendrites. It was found in association with rough endoplasmic reticulum, ribosomes, small vesicles, mitochondria and the plasma membrane including the postsynaptic densities. In the cerebellum, labeling was observed in several types of neuron such as granule, Golgi, basket and Purkinje cells. In contrast to the cerebrum, immunoreactivity was accentuated in the perikarya or axon terminals, and the synaptic vesicles represented another organelle that was immunopositive for CD38. In both of these CNS regions, the nuclear envelope, particularly the outer membrane, showed constant labeling. Diffuse immunoreactivity was also present in the astrocytes from the perikarya to the processes including the perivascular glia limitans. Oligodendrocytes and microglia were immunonegative for CD38. The pattern of distribution of CD38 in the CNS is suggestive of multiple roles for this molecule at various functional sites in both neurons and astrocytes.