The purpose of this study is to determine if surgical approach to the inner ear is feasible without generating a hearing loss in an animal model. Five Macaca fascicularis were used as experimental animals and followed up for 27 months. Mastoidectomy, posterior tympanotomy and promontorial cochleostomy were performed on four specimens and one specimen was kept as control animal. Before and after drilling and exposing the endosteal layer and the membranous labyrinth, otoacustic emissions (dPOAE) and auditory brainstem responses (ABR) were used to test hearing. In vivo experimental studies prove it is reliable to expose the membranous labyrinth without causing hearing loss. dPOAE were present after 3, 6, 12, 24 and 26 months of follow-up. Regarding the ABR results from the four M. fascicularis in which a cochleostomy has been carried out, auditory thresholds are within the 20-30 dB interval at 27 months of follow-up. Experimental studies support clinical experiences indicating it is feasible to surgically approach the membranous labyrinth of the cochlea without damaging its hearing function.