The ototoxicity of ethacrynic acid and furosemide is studied in cats utilizing round window electrodes and light microscopy. Both diuretics produce a primary depression of the cochlear microphonic and action potential with almost complete recovery in one hour. Depending upon the dosage and method of administration a severe secondary depression may develop with ethacrynic acid. This delayed depression is associated with the appearance of outer hair cell degeneration in the basal and middle coils of the cochlea.