The rat is an altricial animal and is thus a useful model for the study of auditory development. The endocochlear potential (EP) undergoes a rapid increase in magnitude from the end of the 1st week to the beginning of the 3rd postnatal week. The purpose of this study was to examine the ultrastructure of the developing stria vascularis in the rat pup in order to correlate functional changes with structural alterations. Rat pups of various ages underwent EP measurement under Rompun anesthesia. The cochleas were rapidly removed under deep pentobarbital anesthesia. The tissues were fixed in 2.5% glutaraldehyde and postfixed in 1.5% osmium tetroxide. Thin sections were viewed and photographed using a Hitachi H7000 transmission electron microscope. A series of distinct developmental changes were observed. Intermediate and basal cells became more distinct from one another, and basal cells became more elongated. Marginal cells underwent progressive development of basolateral infoldings. These cytologic changes may signal the development of ion transport mechanisms necessary for EP development.