The exact mechanism of ototoxicity by cisplatin remains obscure. In the present study, the toxicity of cisplatin metabolites was evaluated using isolated outer hair cells of guinea pig cochlea. Cisplatin metabolites were produced by reaction of cisplatin and S9 fraction from guinea pig liver after administration with phenobarbital. The viability of the outer hair cells did not decrease after the incubation in non-metabolized cisplatin solutions or S9 fraction only, demonstrating the absence of toxic effect from non-metabolized cisplatin and the absence of intrinsic toxic substances in S9 fraction. However, metabolized cisplatin reacted with S9 fraction at 37 degrees C significantly decreased the viability. Cisplatin reacted with heat-treated S9 fraction did not produce a cytotoxic effect, suggesting an enzymatic basis for the formation of metabolites. When NADP was omitted, outer hair cells remained viable, suggesting the significant role of oxygenases in the metabolism of cisplatin. The existence of a toxic metabolites could account for the poor correlation between drug peak level and pathologic damage to the inner ear, and a delay in the onset of ototoxicity could represent the time necessary for induction of the metabolizing enzymes or for reaching a critical point in toxicity by metabolites.