Cis-dichlorodiammine platinum, an agent with antineoplatic activity, was examined by otototic effects in guinea pigs. Otototoxicity was evaluated by suppression of the cochlear potentials, alteration of electrolyte concentrations in the cochlear fluids and by histopathological changes in the inner ear. Guinea pigs treated with ip injection of cis-dichlorodiammine platinum 1.5 mg/kg (5 doses/wk) developed marked suppression of sound-evoked cochlear responses but showed little changes in the endocochlear potential. The electrolyte concentrations of the cochlear fluids were not substantially altered in cis-dichlorodiammine platinum-treated guinea pigs.