Hair cells in the mammalian cochlea transduce mechanical stimuli into electrical signals leading to excitation of auditory nerve fibres. Because of their important role in hearing, these cells are a possible site for the loss of cochlear sensitivity that follows acoustic overstimulation. We have recorded from inner and outer hair cells (IHC, OHC) in the guinea pig cochlea during and after exposure to intense tones. Our results show functional changes in the hair cells that may explain the origin of noise-induced hearing loss. Both populations of hair cells show a reduction in amplitude and an increase in the symmetry of their acoustically evoked receptor potentials. In addition, the OHCs also suffer a sustained depolarization of the membrane potential. Significantly, the membrane and receptor potentials of the OHCs recover in parallel with cochlear sensitivity as measured by the IHC receptor potential amplitude and the auditory nerve threshold. Current theories of acoustic transduction suggest that the mechanical input to IHCs may be regulated by the OHCs. Consequently, the modified function of OHCs after acoustic overstimulation may determine the extent of the hearing loss following loud sound.