In order to study the linearity of outer hair cell fast electromotility in vivo, an acoustic tone was used to interact with the electrically evoked distortion product otoacoustic emissions. Otoacoustic emissions at the primary frequencies (f1, f2, where f1 < f2) and the distortion frequencies (2f1 - f2 and f2 - f1) were evoked by a complex current, with f1 and f2 components, delivered to the gerbil round window. An externally given acoustic tone at the frequency f1 or f2 with appropriate phase and level can completely abolish both the 2f1 - f2 and f2 - f1 distortion tones. Because the external tone causes basilar membrane vibration at its natural topographic locations, this result indicates that the observed distortion tones were generated near the locations of f1 and f2 frequencies on the basilar membrane and that no distortion occurred from the stimulated cells near the electrode. The study strongly suggests a linear electromechanical transduction of the outer hair cells in the sensitive cochlea.