It is well known that the anti-cancer drug cisplatin has an ototoxic property; however, the details are not yet evident. In this study, the expression of inducible nitric oxide synthase (iNOS/NOS II) in the vestibule of guinea pigs after i.p. injections of cisplatin was examined immunohistochemically. Three days after the injection of cisplatin (10 mg/kg) or placebo, animals were sacrificed. Then the temporal bones were removed and subjected to immunohistochemical studies for iNOS. In the cisplatin group, iNOS was detectable, whereas the tissue in the control group was negative for iNOS. The vestibule, the wall of blood vessels and the vestibular ganglion cells showed immunoreactivity for iNOS. It is known that iNOS catalyzes an inadequate quantity of NO under pathological conditions. Increased NO levels lead to inner ear dysfunction. Therefore, our results indicate that iNOS could also mediate the vestibulo-toxicity of cisplatin.