Immunocytochemistry has been used to assess the distribution of the mitosis-associated protein kinase, cdc2, in mature neurons of rat brain. A robust signal is apparent in most, but not all neurons in cerebellum, cortex and hippocampus. The signal is concentrated in neuronal nuclei with lower levels of immunoreactivity in neuronal cytoplasm. High resolution analysis indicates that the amount of cdc2 is uneven, suggesting that regulated amounts of enzyme may reflect different functional status of the neurons. In many cases, the kinase is concentrated at the nuclear periphery, implying that it may be particularly involved in phosphorylating substrates in this subnuclear compartment. These results indicate additional roles for cdc2 in terminally differentiated neurons which have undergone final mitosis.