We have shown recently that isolated cochlear outer hair cells change their axial stiffness when their membrane potential is altered under voltage-clamp. Here we extend those observations, using a more stable mechanical platform, the microchamber, to hold the cells and to deliver voltage commands. Cell stiffness is determined by opto-electronically measuring the amplitude of motion of a flexible fiber as it is loaded by the cell. Cell stiffness is decreased by depolarization and increased by hyperpolarization. The stiffness changes have been measured with sinusoidal electrical command signals up to 1750 Hz and fiber motion up to 2000 Hz. It is shown that electrically evoked stiffness changes and length changes (electromotility) have very similar characteristics and may arise in a common process.