Auditory function in the mammalian inner ear is optimized by collaboration of two classes of sensory cells known as inner and outer hair cells. Outer hair cells amplify and tune sound stimuli which are transduced and transmitted by inner hair cells. Although they subserve distinct functions, they share a number of common properties. Here we compare the properties of mechanotransduction and adaptation recorded from inner and outer hair cells of the postnatal mouse cochlea. Rapid outer hair bundle deflections of ~0.5 microns evoked average maximal transduction currents of ~325 pA, whereas inner hair bundle deflections of ~0.9 microns were required to evoke average maximal currents of ~310 pA. The similar amplitude was surprising given the difference in the number of stereocilia, 81 for outer hair cells and 48 for inner hair cells, but may be reconciled by the difference in single channel conductance. Step deflections of inner and outer hair bundles evoked adaptation that had two components: a fast component that consisted of ~60% of the response occurred over the first few msec and a slow component that consisted of ~40% of the response followed over the subsequent 20-50 msec. The rate of the slow component in both inner and outer hair cells was similar to the rate of slow adaptation in vestibular hair cells. The rate of the fast component was similar to that of auditory hair cells in other organisms and several properties were consistent with a model that proposes calcium-dependent release of tension allows transduction channel closure.