The nature of the ototoxic action of various antibiotics has been investigated since Hinshaw and Feldman(1945) was described the toxic effects of dihydrostreptomycin. Ototoxicities of kanamycin, viomycin, neomycin, polymyxin and colimycin were proved by many investigators up to date. Especially, colistin, used in this study, was discovered by Koyama in 1950 and it was produced from Bacillus colistinnus, and organism isolated from the soil. Colistin is a polypeptide that is similar but not identical with the polymyxins. This promising drug has been shown to be active primarily against gram negative bacilli and the pharmacological action of colistin is considered to be bacteriocidal and bacteriostatic. Thus the colistin has been a very useful drug to gram negative infection and ototopically applicated in clinical use. The ototoxicity of colistin was discussed by Kohonen and Tarkkanen in 1969 but their report was not satisfactory. In this paper cochlear damage resulting from intratympanic application of colistin was experimentally studied in guinea pigs. The hair cells of cochlea were histopathologically examined according to the method of surface preparation using osmium tetroxide and considered the relationship between cochlear damage and applicable duration. Generally, within 10 days, cochlear damage is limited in outer hair cells of the basal turn and which shows slight blurring and distorsion of the cell membrane. Within 20 days, the damage is moderate except apical turn but damage is still limited in outer hair cells. Within 30 days, outer hair cells are completely damaged in all cochlear turns and also blurring, distorsion, shrinkage, and degeneration. Consequently, otolaryngologist might be use the lower concentrated ototoxic antibiotics within 10 days in case of clinical use.