The function of the hearing organ is based on mechanical processes occurring at the cellular level. The mechanical properties of guinea-pig isolated sensory cells were investigated using two different techniques. The stiffness of the outer hair cells along the longitudinal axis was measured by compressing the cell body using stiffness-calibrated quartz fibres. For cells with a mean length of 69 micron, the mean axial compression stiffness was 1. 1+/-0.8 mN/m (+/-SD). There was an inverse relation between stiffness and cell length. The stiffness of the cell membrane perpendicular to the longitudinal axis of the sensory cell was measured by indenting the cell membrane with a known force. The mean lateral indentation stiffness was 3.3+/-1.5 mN/m (+/-SD) for cells with a mean length of 64 microm. Longer cells were less stiff than short cells. Modelling the hair cell as a shell with bending resistance, finite element calculations demonstrated that the axial compression stiffness correlated well with the lateral indentation stiffness, and that a simple isotropic model is sufficient to explain the experimental observations despite the different stress strain states produced by the two techniques. The results imply that the two different stiffness properties may originate from the same cytoskeletal structures. It is suggested that the mechanical properties of the outer hair cells are designed to influence the sound-induced motion of the reticular lamina. In such a system, stiffness changes of the outer hair cell bodies could actively control the efficiency of the mechanical coupling between the basilar membrane and the important mechanoelectrical transduction sites at the surface of the hearing organ.