Most current theories of cochlear mechanics assume that the pattern of cochlear partition vibration is simple, similar to that of a bending beam. Recent evidence suggests, however, that the vibration of the organ of Corti can be complex and that multiple vibrational modes may play an important role in cochlear transduction. Inner hair cell (IHC) and auditory nerve responses to pure tones can exhibit large phase shifts and complex response waveforms with increasing stimulus level. In contrast, the comparable basilar membrane (BM) responses are much less complex, exhibiting only small phase shifts and relatively sinusoidal waveforms. To reconcile the differences observed between the published BM data and the IHC data, we have recorded receptor potentials from IHCs and compared these waveform data to the output of two computational models:a traditional linear model where IHC excitation depends only on BM displacement and a new model that assumes that outer hair cell (OHC) force production provides the major mechanical input to the IHC along with two additional mechanical components. Comparisons of the output of the two models with the experimental data show that the new model is capable of reproducing the very complex voltage responses of the IHC recorded in vivo whereas the traditional model performed poorly. (C) 1999 Published by Elsevier Science B.V. All rights reserved.