This study focused on the immunohistochemical identification of the beta/A4 amyloid precursor protein (APP) in various developmental stages of both the rat central nervous system (CNS) and the peripheral nervous system (PNS). A comparative study with myelin basic protein (MBP) and synaptophysin (SYP) facilitated the understanding of neuronal maturation and synaptogenesis on both prenatal and postnatal development. Our immunohistochemical study revealed APP to be widely distributed through the nervous system while existing mainly in the cytoplasm, dendrites and axons of the neurons. However, immunoreactivity was also observed in either the ependymal cells or the choroid plexus epithelial cells. Our immunostaining was carried out by the hydrated autoclaving method and revealed the expression of APP at embryonic day 15 in the neuron of the mesencephalic nucleus of the trigeminal nerve and the anterior horn of the spinal cord, trigeminal and spinal ganglion, ependymal cells and the choroid plexus. We thus observed dramatic changes of APP expression in the cerebellum from the embryonic stage. The maturation of synaptogenesis in the cerebellar molecular layer was parallel to the extension of the dendrites of Purkinje cells, which revealed immunoreactivity for APP. These findings suggested that APP played an important role in neuronal maturation and synaptogenesis. Thus, APP is considered to be a useful marker for neuronal development.