Electromotility, i.e., the ability of cochlear outer hair cells (OHCs) to contract and elongate at acoustic frequencies, is presumed to depend on the voltage-driven conformational changes of "motor" proteins present in the OHC lateral plasma membrane. Recently, two membrane proteins have been proposed as candidates for the OHC motor. A sugar transporter, GLUT-5, was proposed based on its localization in the OHCs and on the observation that sugar transport alters the voltage sensitivity of the OHC motor mechanism. Another candidate, "prestin," was identified from a subtracted OHC cDNA library and shown to impart voltage-driven shape changes to transfected cultured cells. We used antibodies specific for these two proteins to show that they are highly expressed in the lateral membrane of OHCs. We also compared the postnatal expression patterns of these proteins with the development of electromotility in OHCs of the apical turn of the rat organ of Corti. The patch-clamp recording of transient charge movement associated with electromotility indicates that half of the maximal expression of the motor protein occurs at postnatal day 9. Prestin incorporation in the plasma membrane begins from postnatal day 0 and increases progressively in a time course coinciding with that of electromotility. GLUT-5 is not incorporated into the lateral plasma membrane of apical OHCs until postnatal day 15. Our results suggest that, although GLUT-5 may be involved in the control of electromotility, prestin is likely to be a fundamental component of the OHC membrane motor mechanism.