Repeated i.d. injection doses of endotoxin lipopolysaccharide of E. coli at certain time intervals were found to give rise to the following changes in the inner ear in mice: In the organ of Corti of the cochlea, there were noted degeneration and disappearance of the inner and outer hair cells of the lower turn, partly with a partial disappearance of nerve fibers in the osseous spiral lamina. Degenerative changes seen in the upper turn were of milder degree. The stria vascularis exhibited enlarged intercellular spaces, the appearance of lysosomes in the constituent cells, and degeneration thereof. Changes observed in the vestibular organ were varying degrees of degeneration of types I and II sensory cells of the crista ampullaris and macula as well as a partial disappearance of these cells. The dark cells were found to have undergone such changes as the formation of vacuoles of varying size and widened interspaces between infoldings, thus providing presumptive evidence of a change in the composition of the endolymph. These observations suggest the possibility that Shwartzman reaction might be another probable cause of inner ear deafness or equilibrium disturbances in actual clinical cases which are at present claimed to be of unknown etiology.