The effects of ethacrynic acid, a potent new diuretic which causes sodium and potassium excretion, on the inner ear were studied by means of phase contrast microscopy and electron microscopy. High doses of ethacrynic acid were administered intra-peritoneally to mice until the Preyer’s reflex disappeared. Some of the animals were sacrificed immediately after final administration of ethacrynic acid and the others were allowed to recover for 1 to 4 months. The drug mainly affected the marginal cells of the stria vascularis. This structural change in the stria vascularis has recovered after 1 to 4 months. The organ of Corti was less affected than the stria vascularis but degeneration appeared in the outer hair cells in the lower turn, starting with degenerations in the mitochondria and distortion of stereocilia which did not recover. In the vestibule, vacuolization and protrusion of the supra-nuclear cytoplasm were observed in both the type I and type II hair cells. The nerve chalice was swollen and its membrane and the cytoplasmic membrane of the type I hair cell were irregularly separated. The ototoxic mechanism of this drug to the inner ear sensory epithelia and the transient hearing loss were discussed from the morphological point of view.