An experiment was performed on 31 guinea pigs to study the effect of fistulae of the horizontal canal, superior canal, common crus, and utricle on the course of experimentally induced endolymphatic hydrops. The effect of fistulae on corresponding parts of normal ears was also studied in an additional 17 animals. The results indicated a remarkable consistency in healing of the fistulae in both groups of animals. Fistulae had no significant effect on the course of endolymphatic hydrops, irrespective of whether the fistulae were made immediately before or several months after obliteration of the endolymphatic duct or whether a polyethylene strut was introduced into the membranous labyrinth. Once again, the functional significance of the endolymphatic sac was clearly apparent, for ablation of the sac consistently produced severe hydrops and atrophic changes in sensory and neural structures. Assuming that the mechanisms of labyrinth repair in the guinea pig are comparable to the higher mammalian ear, it is apparent that procedures designed to surgically fistulize the membranous labyrinth are of questionable value and are probably not rational therapeutic approaches to the management of Ménière's disease.