It has recently been proposed that elements which contribute to active cochlear processes develop at the same time (between postnatal day (PND) 12 and 21) as the shift of the place code in the developing gerbil cochlea. Since outer hair cells (OHCs) have been implicated in these processes, we have hypothesized that developing OHCs will exhibit changes in anatomical features that contribute to cochlear maturation. Our results demonstrate that the ultrastructural characteristics of OHCs change after the onset of hearing (PND 12), during the time that cochlear nonlinearities are being established (PND 12-21). Differences are primarily associated with the distribution of cytoplasmic organelles. The subsurface cisternae (SSC), which are thought to be related to the mechanical support of the outer hair cell, to cell motility, and therefore to cochlear mechanics, are present at PND 10 but remain immature, with cisternal layers added during the preweanling period. In immature OHCs, more mitochondria are centrally-located than in mature OHCs. During development mitochondria come to form a continuous row near the innermost leaflet of the SSC. These ultrastructural features undergo rapid change during the maturation of peripheral auditory function.