The right vestibular aqueduct was obliterated in five guinea pigs and the inner ear was examined histologically after three weeks. No distension of the endolymphatic space was observed in the normal left inner ear in all animals. Vestibular aqueducts were oblitercted with new bone formation, and distension of the endolymphatic space was noted in the cochlear and the saccule in all animals. Cochlear hydrops was more pronounced in the apical area and upper basal turns with adhesion of the Reissner membrane to the bony wall of the scala vestibuli. Vestibular changes were characterized by marked dilatation of the saccular wall with adhesion of the membrane to the endosteal membrane of the footplate, and the utricle was slightly distended. Spiral ganglion cells were degenerated in the apical area in one animal. The stria vascularis and hair cells were grossly normal in appearance.