To investigate the morphological changes of the nuclei in the hair cells of the receptor organ in the cochlea, cytological observation was carried out on the hair cells following exposure of 90 phone with or without interruption of the ossicular chain of the guinea pigs. Results obtained were summerized as follows: 1) The significant changes in the receptor organ of the cochlea following exposure of 90 phone were noted in the outer hair cells in both experimental groups with or without interruption of the ossicular chain. 2) In the experimental group with interruption of the ossicular chain, the mild pycnosis and the irregular arrangements were noted in the nuclei of the outer hair cells of the receptor organ in the cochlea. 3) In the experimental group without interruption of the ossicular chain, the severe pycnosis and the disappearance of the nuclei were noted in the outer hair cells in the corti's organ of the cochlea.