Calmodulin is a small, acidic, calcium-binding protein thought to regulate many cellular functions. In the brain of the adult mouse, calmodulin was found immunohistochemically to localize mainly in the neurons. In the developing brain, the immunoreactivity to anti-calmodulin antibody appeared early in the cells in the low brain stem but late in the cerebral cortex, hippocampus, and cerebellum, except for the deep cerebellar nuclei. The cells in the major proliferative layer present during early development, such as the matrix cells in the cerebral cortex and the cells in the external granular layer in the cerebellum, did not show the immunoreactivity. In the cerebral cortex, the migrating cells and the cells in the cortical plate were also negative while the deep cortical cells, which had probably settled in their final position, became positive. The comparison of these results with the immunohistochemical appearance of neuron specific enolase, a characteristic protein in the brain, suggested that calmodulin appeared with some maturation of the neurons as neuron specific enolase.