The compound action potential (AP) to a broad-hand click stimulus, recorded from the round window in guinea-pigs and from the promotory in humans, was suppressed by a tonal stimulus. The degree of suppression depended upon the relative levels of the click and masking tones and upon the tone frequency. Using a fixed 50% suppression criterion, the tone-on-click masking revealed that for near threshold click levels a relatively narrow population of nerve fibres contributed to the AP. Both for threshold and suprathreshold clicks the population of neurones contributing to the AP was centred at the minimum of the AP audiogram with no shift toward higher frequency as the click level was raised. The shape of the response area revealed by tone-on-click masking is similar to that shown by tone-on-tone masking. Application of the tone-on-click masking technique to Kanamycin poisoned guinea-pigs and to humans with cochlear lesions indicated that useful information about residual hearing could be obtained even at very low frequencies.