The effective antineoplastic agent cisplatin has serious side effects, such as oto- and nephrotoxicity that limit its application and the patients healthy life. Ototoxicity is a problem of special importance in children, because deafness hampers their language as well as psychosocial development. Recently, organic cation transporters (OCTs) were identified in vitro as cellular uptake mechanisms for cisplatin. In the present study, we investigated in an in vivo model the role of OCTs in the development of cisplatin induced oto- and nephrotoxicity. The functional effects of cisplatin treatment on kidney (24 hr excretion of glucose, water, and protein and apoptosis) and hearing (auditory brainstem response, ABR) were studied in wild type (WT) and OCT1/2 double knock-out (KO) mice. While in WT cisplatin led to reduced ABR and increased renal glucose, water, and protein excretion and apoptosis, no sign of ototoxicity and only mild nephrotoxicity was observed after cisplatin treatment of KO mice. Co-medication of WT mice with cisplatin and the organic cation cimetidine protected completely from ototoxicity and partly from nephrotoxicity. For the first time we showed that OCT2 is expressed in the Corti organ on the hair cells and in the stria vascularis of the cochlea and that it plays a critical role in cisplatin-induced ototoxicity. Furthermore, cisplatin sensitive cell lines from paediatric tumors showed no expression of mRNA for OCTs, indicating the feasibility of a therapeutical approach aimed to reduce cisplatin toxicities by competing OCT2-mediated cisplatin uptake in renal proximal tubular and cochlear hair cells with another organic cation. These findings are very important to establish chemotherapeutical protocols aimed to maximize the antineoplastic effect of cisplatin, while reducing the risk of toxicities. Supported by the Deutsche Krebshilfe, Grant 108539