Pendred syndrome is due to loss-of-function mutations of Slc26a4, which codes for the HCO₃- transporter pendrin. Loss of pendrin causes deafness via a loss of the K⁺ channel Kcnj10 in stria vascularis and consequent loss of the endocochlear potential. Pendrin and Kcnj10 are expressed in different cell types. Here we report that free radical stress provides a link between the loss of Kcnj10 and the loss of pendrin. Studies were performed using native and cultured stria vascularis from Slc26a4^+/- and Slc26a4^-/- mice as well as CHO-K1 cells. Kcnj10, oxidized proteins and proteins involved in iron metabolism were quantified by Western blotting. Nitrated proteins were quantified by ELISA. Total iron was measured by ferrozine spectrophotometry and gene expression was quantified by qRT-PCR. At postnatal day 10 ( P10), stria vascularis from Slc26a4^+/- and Slc26a4^-/- mice expressed similar amounts of Kcnj10. Slc26a4^-/- mice lost Kcnj10 expression during the next 5 days of development. In contrast, stria vascularis, obtained from P10 Slc26a4^-/- mice and kept in culture for 5 days, maintained Kcnj10 expression. Stria vascularis from Slc26a4^-/- mice was found to suffer from free radical stress evident by elevated amounts of oxidized and nitrated proteins and other changes in protein and gene expression. Free radical stress induced by 3-morpholinosydnonimine-N-ethylcarbamide (SIN-1) was found to be sufficient to reduce Kcnj10 expression in CHO-K1 cells. These data demonstrate that free radical stress provides a link between loss of pendrin and loss of Kcnj10 in Slc26a4^-/- mice and possibly in human patients suffering from Pendred syndrome.