Osmolarity changes in inner ear fluids have long been considered to be contributing factors to Menière's disease. Our present study demonstrates that small changes in the osmolarity of a surrounding in vitro medium induce fast contractions (hypo-osmotic solution) or elongations (hyperosmotic solution) in isolated outer hair cells of the guinea pig. These changes were reversible upon returning cells to iso-osmotic conditions. Up to five cycles of shape change could be sustained by these cells without obvious detriment to their morphology. These findings suggest that fluctuant changes in osmolarity of inner ear fluids can result in similar fluctuant changes of hair cell shape. Since the outer hair cells may control cochlear micromechanics and function by their active motility, osmotically induced abnormalities of cell dimensions and motility may contribute to the audiological manifestations of fluctuant hearing loss.