Period histograms of the evoked activity from the cells of the guinea pig spiral ganglion were collected in response to a 40-Hz sinusoid plus a high-frequency tone corresponding to the characteristic frequency (CF) of the cells (13–16 kHz). Period histograms were synchronized to the 40-Hz tone. Phase of the 40-Hz motion of the basilar membrane was inferred from cochlear microphonic. At moderate intensities of the 40-Hz tone (80–90 dB SPL), suppression of the CF response was observed for basilar-membrane displacements towards scala tympani and potentiation towards scala vestibuli. For higher 40-Hz intensities (90–100 dB SPL), suppression of the CF response was observed for displacements of the basilar membrane towards scala tympani and also for maximum displacements towards scala vestibuli. Potentiation was still observed for moderate displacements towards scala vestibuli. For even higher intensities of the 40-Hz tone, the neural response to the 40-Hz tone predominated. Such modulation is observable for high-frequency tones of frequency greater than 1 octave below CF. For frequencies lower than this, no modulation was observed. [Work supported by the National Health and Medical Research Council.]