Cerebrospinal fluid pressure changes are reflected in perilymph via the cochlear aqueduct and are passed on to the endolymph, probably via Reiss-ner's membrane. The hypothesis that the endolymph pressure is transmitted by the cerebrospinal fluid via the endolymphatic sac and duct appears to be refuted by our results. In the normal labyrinth the peri-lymphatic and endolymphatic pressure are equal within the accuracy of our measurements. The effective cross section of the cochlear aqueduct together with the expansion properties of the round window membrane may serve as a protective mechanism against sudden changes in the cerebrospinal fluid. A pressure gradient between endolymph and perilymph will have to stay below 2 cm of water, a value at which rupture is to be expected. Therefore, damage to the sensory cells due to pressure in case of an endolymphatic hypertension is very improbable, provided the perilymphatic pressure is not affected, which is likely when the cochlear aqueduct is open. The expansion of the endolymphatic compartment in a relatively short time appears to go as far as four times it's original volume after which rupture occurs.