The combination of 10 mg/kg ethacrynic acid (ETA) and 100 mg/kg kanamycin (KA) caused neither morphologic damage to the cochlea nor change in the auditory brain stem response of the chinchilla. However, after pretreatment with a single dose of buthionine sulfoximine (BSO; 800 mg/kg intraperitoneally) to reduce intracellular glutathione (gamma-glutamylcysteinylglycine; GSH) levels, the above single administration of ETA and KA resulted in complete deafness and severe morphologic damage. The kidney, which has a rapid GSH turnover and is therefore especially susceptible to GSH depletion by BSO, also demonstrated severe damage after this treatment. A similar rapid turnover of GSH and resulting limited capacity to detoxify reactive metabolites and free radicals may determine cochlear and renal vulnerability to this toxicity. These findings may explain the clinical observations of enhanced ototoxicity in patients administered amino-glycoside antibiotics concomitantly with loop diuretics.