The temporal bones of a patient who suffered sudden deafness and ataxia after administration of both furosemide and ethacrynic acid, were prepared for light and electron microscopy. There was no loss of hair or supporting cells. However, some hair cells, in both the vestibular neuroepithelium and the organ of Corti, particularly in the basal turn, were more densely staining and more granular than normal. Membrane whorls also were common within mitochondria of such cells. The endoplasmic reticulum of some spiral ganglion cells was dilated. The major cytologic changes were found in the stria vascularis of the cochlea and dark cell areas of the vestibular system. There was marked dilatation of the intercellular fluid spaces, consistent with the biochemical observation that loop diuretics interfere first with enzyme systems responsible for fluid transport within the inner ear.