The tuning properties of the basilar membrane and the presence of acoustic emissions from the cochlea suggest that an energy consuming, mechanically active cochlear amplifier exists. Some models of this amplifier demand a mechanical resonator within the cochlea. The lengths of the stereocilia of the inner and outer hair cells in man and the guinea pig have been measured from scanning electron micrographs using a stereometric technique. In both species there is a linear increase in the length of the longest inner hair cell stereocilia with distance along the cochlea. There are, however, marked differences between the dimensions of the outer hair cell stereocilia in the two species. In man, there is an increase in length which is is a hyperbolic function of distance along the cochlear duct. The picture is more complicated in the guinea pig. This could account for some of the differences in auditory physiology between the two groups. The mechanical resonance properties of the human OHC stereocilia have been assessed, and, with certain assumptions, these properties are such that resonance of the stereocilia of the OHCs could form part of the cochlea amplifier, at least in man.