We observed the distribution pattern of Ca2+/calmodulin-dependent protein kinase IV in rat brain and spinal cord using an immunohistochemical method by light and electron microscopy. Particularly strong immunoreactivity was detected in the telencephalic structures such as the olfactory bulb, cerebral cortex, hippocampal formation, caudate-putamen, most nuclei of the dorsal thalamus and the granule cell layer of the cerebellum. Relatively weak staining was observed in the amygdaloid body, some neuron groups of the brainstem reticular formation, the inferior olivary nucleus and the posterior horn of the spinal cord. Immunohistochemical reactivity was not detected in the globus pallidus, substantia nigra, sensory and motor nuclei of the cranial nerves, or in the spinal cord anterior horn. Overall, the distribution of Ca2+/calmodulin-dependent protein kinase IV-like immunoreactivity broadly paralleled the sites of expression of signals for messenger RNA of this enzyme. At the subcellular level, Ca2+/calmodulin-dependent protein kinase IV-like immunoreactivity appeared exclusively in the nuclei of neurons in the various brain regions, and immunopositive reactivity, although less strong, was also observed in dendritic processes, as well as on the granular endoplasmic reticulum in neuronal somata in these areas. Axon terminals, however, did not show immunoreactivity. These studies demonstrate that Ca2+/calmodulin-dependent protein kinase IV-like immunoreactivity is distributed widely in the central nervous system. The significance of the localization of this enzyme in nuclei is discussed in relation to gene expression.