Perilymphatic fistula was produced in guinea pigs by injecting artificial perilymph into the subarachnoid space of the posterior fossa. Rupture of the round window was confirmed by direct observation of the round window membrane under a surgical microscope during the injection. The animals were either vitally fixed or kept alive for one to three months before fixation. Conventional celloidin embedding method was used for serial sectioning. In immediate observation, nine cochleas showed hydrops, seven showed collapse, and 20 of 36 cochleas showed no change. Changes in the vestibular apparatus varied, with collapse of the membranous vestibular labyrinth as the main change. Delayed observation revealed 16 normal cochleas and four collapses and no hydrops out of 20 ears. Of these 20, nine ears showed normal vestibular apparatus, nine collapse, and two saccule hydrops. Loss of the outer hair cells was observed in five of 20 cochleas. Marked compression of the organ of Corti was seen in both immediate and delayed observations.