Intracochlear dc potentials were evoked by stimulating the crossed olivocochlear fibres and were recorded with microelectrodes in 28 cats. From the organ of Corti to the scala media, the dc potentials changed from positive to negative and became larger by a factor of 4–10, reaching −3 mV relative to a reference electrode outside the cochlea. It is concluded that the dc potentials represented postsynaptic activity, generated by crossed olivocochlear fibres impinging on outer hair cells, and that auditory‐nerve fibres are presynaptically inhibited by such activity. New and previous findings on the action of the crossed olivocochlear fibres are discussed in relation to present concepts of summating potentials. The findings that centrifugally and acoustically evoked dc activity interact are taken to indicate that sound‐evoked dc activity in hair cells might regulate activity in auditory nerve fibres. New and previous findings on the uncrossed olivocochlear fibres are discussed. The tentative hypothesis is proposed that these fibres inhibit by their action on the afferent terminals underneath the inner hair cells, and perhaps in no other way.