THE sharp frequency selectivity of single primary auditory neurones in mammals contrasts with the poor mechanical tuning of the basilar membrane 1,2. This discrepancy has generally been explained by postulating some filter mechanism interposed between the basilar membrane and the excitation of auditory afferents2-4. A further property of these neurones which probably cannot be explained by basilar membrane behaviour alone is two-tone inhibition whereby the simultaneous presentation of a second tone reduces the response of a neurone to a first tone at the best, or characteristic frequency5-8. It has been suggested that this inhibition may also be a property of the additional cochlear mechanisms which follow the basilar membrane9. I now present evidence that the loss of sharp frequency selectivity of primary neurones is intimately linked to changes in two-tone inhibition and that the two phenomena might be related to a common mechanism which is vulnerable to perilymph removal. © 1975 Nature Publishing Group.