Mechanical responses to pure-tone stimuli were recorded from the basilar membrane in the basal turn of the guinea-pig cochlea using a displacement-sensitive laser interferometer. The harmonic content of the responses was evaluated using Fourier analysis. 2. Harmonic distortion products were observed in many of the basilar membrane responses. Response components locked to twice the frequency of the stimulus (i.e. 2F0) were the largest of the distortion products. 3. The second harmonic responses showed a bimodal frequency distribution at low to moderate sound pressure levels: one peak occurred around the preparation's best or most sensitive frequency (i.e. when F0 approximately 17 kHz), and another occurred around one-half of the best frequency (when F0 approximately 8.5 kHz). 4. The absolute levels of most distortion products increased progressively with increasing stimulus strength. When expressed with respect to the levels of the fundamental responses, however, the distortion levels usually decreased with increasing stimulus strength. 5. The levels of the distortion decreased (in both absolute and relative terms) with deterioration in the physiological condition of the cochlea. 6. Maximum second harmonic distortion levels amounted to approximately 3.5 and approximately 28 % of the fundamental responses to tones near and below the best frequency, respectively. 7. The above findings are shown to be consistent with a highly simplified model of cochlear mechanics which incorporates an asymmetric, saturating non-linearity in a positive feedback loop.
