Experimental endolymphatic sac surgery was performed in 35 guinea pigs to evaluate the changes in cochlear function and structure caused by surgical manipulations which included incising the sac, aspirating the endolymph and probing the vestibular aqueduct. The experimental data showed that the changes in the cochlear potential, the auditory reaction threshold and the cochlear morphology in some guinea pigs were induced by these manipulations, especially aspirating the endolymph or probing the vestibular aqueduct. The possible mechanisms of the cochlear impairment may be damage caused by positive or negative pressure, the change in endolymphatic environment and the secondary inner ear hydrops. The experimental findings indicate that the cochlear function and structure can be affected by surgical manipulations, especially aspirating the endolymph or probing the vestibular aqueduct. Therefore, these manipulations should be avoided during the surgical procedure.