Mercury is known to be ototoxic, however the exact site and mechanism of the toxic effects on the organ of Corti are still obscure. The present experimental study was carried out to observe such toxic effects on the hair cells after administration of mercury chloride in guinea pigs. Ten healthy guinea pigs weighting from 200 to 300 grams were used. Following administration of 1% mercury chloride(50 mg/kg/B.W.), the animals were sacrificed and the cochlea was fixed with 1% osmic acid. Morphological findings of the hair cells were observed with the surface preparation of the cochlea, and following results were obtained. The significant changes in the receptor organ of Corti were mainly noted in the hair cells between the upper portion of the basal turn and the lower portion of the second turn of the cochlea. The nuclei of the outer hair cells were more markedly changed than the inner hair cells. In electronmicroscopic study, degenerative changes such as vesicles and lysosome in cytoplasm and fusion of stereocilia were observed.