Cis-diamminedichloroplatinum [II] is a potent anti-cancer platinum compound which has the property of inhibiting cell division but not cell growth. This agent, however, has many side effects including renal damage and ototoxicity. Cis-diamminedichloroplatinum [II] induced cochlear degeneration in the rats was studied by transmission electron microscopy. Sprague-Dawley strain rats each weighing 200-220gm and hearing normal Preyer reflexes were divided into three groups. Group I was given 2mg/kg/day, Cis-diamminedichloroplatinum [II], intraperitoneally for 6 consecutive days(total dosage, 12mg/kg). Group II was given a dose of 2mg/kg/day for 10 consecutive day(total dosage, 20mg/kg). Group III was given 4mg/kg/day for 6 consecutive days(total dosage, 24mg/kg). To evaluate inner ear damage, the animals were sacrified 24 hour after the administration of Cis-diamminedichloroplatinum [II]. From electron microscopic observation in this study, the degeneration of the organ of Corti started sporadically in almost every outer hair cells with a strong prevalence in the outer hair cell in the basal turn and inner hair cells rarely damaged. The first row of the outer hair cells tended to be more severely damaged. No distinct starting point for the degeneration of the organ of Corti could be found. The susceptibility to cochlear damage by Cis-diamminedichloroplatinum [II] was dose dependent. Electron microscopic study in the outer hair cells showed denaturation of the mitochondria, various vacuoles, dispersed endoplasmic reticulum, rupture of the hair cell and leaking of the cell constituents into endolymphatic space, increased lysosomes and fusion of hair cell with Deiter's cell. In the stria vascularis, swollen and atrophied marginal cell with various vacuoles appeared.