The purpose of this study was to investigate the developmental change of epithelial Na channel function in epithelial cells of rat Reissner's membrane. The Reissner's membrane was isolated from the upper turn of the cochlea at different stages of development. The vibrating probe technique was chosen to measure transepithelial currents under short circuit conditions. Amiloride-sensitive Na current in the Reissner's membrane developed postnatally between days 3 and 5, which was completely blocked by amiloride. This tendency remained from days 5 through 14. Development of Na transport in epithelial cells of the Reissner's membrane may contribute considerably to the accomplishment of low Na concentration in the endolymph during early neonatal period.