By using polyclonal antibodies to C- and N-terminal portions of beta-amyloid precursor protein (APP), we immunohistochemically examined postnatal and adult rat brains. Immunoreactivity was observed in myelin of nerve fibers in myelination and of mature myelinated fibers, as well as in various brain cells. In trigeminal nerve roots, the immunoreactivity was observed in central myelin but not in peripheral myelin. The results strongly suggest that APP is a component of central myelin in rat.