It is thought that the sensitivity of mammalian hearing depends on amplification of the incoming sound within the cochlea by a select population of sensory cells, the outer hair cells. It has been suggested that these cells sense displacements and feedback forces which enhance the basilar membrane motion by reducing the inherent damping of the cochlear partition. In support of this hypothesis, outer hair cells show membrane-potential-induced length changes at acoustic rates. This process has been termed 'reverse transduction'. For amplification, the forces should be large enough to move the basilar membrane. Using a displacement-sensitive interferometer, we tested this hypothesis in an isolated cochlea while stimulating the outer hair cells with current passed across the partition. We show here that the cochlear partition distorts under the action of electrically driven hair cell length changes and produces place-specific vibration of the basilar membrane of a magnitude comparable to that observed near auditory threshold (about 1 nm). Such measurements supply direct evidence that cochlear amplification arises from the properties of the outer hair cell population.