The present study was undertaken to determine the target sites of polymyxin B ototoxicity. This drug, at the concentration of 1 mM, was perfused through the scala tympani of the guinea pig cochlea, and cochlear microphonics and endocochlear potentials were monitored. Both cochlear potentials altered but in an independent manner. These findings indicated that not only the organ of Corti but also another tissue is involved in the ototoxicity produced. The best locus for this is the vascular stria.