Exposure to large dosages of otoxic antibiotic drugs is known to cause hair cell damage in the organ of Corti. This has been studied both clinically and in animal experiments and it has been established that the sensory cell and subsequent neural degeneration are the main histologic findings. In this paper, using guinea pigs as experimental animals, modes of sensory cell damage by kanamycin intoxication were observed. As a results, sensory cell degeneration was frequently observed in the outer hair cells of all cochlear turns and was severe in the basal turn. However in the apical turn, damage of the inner hair cells was observed. And in the 3 week experimental groups, sensory cell degeneration was almost not observed or minimal. However sensory cell degeneration of the 6 week experimental groups was more severe. The electron microscopic examination revealed mild proliferation and degeneration of mitochondria, proliferation of the endoplasmic reticulum, vesicles, vacuoles in the cytoplasm and appearance of lysosomes. The degree of severity in alteration of fine structures was in proportion to the injected amount of kanamycin.