We analysed the fluid pressure in the perilymphatic and endolymphatic spaces of the cochlea in eight guinea pigs after endolymphatic hydrops (EH) was surgically created. The control experiments were done in the opposite ears. EH was histologically confirmed after the conclusion of the pressure measurements. No statistically significant pressure difference between the scala media and the scala tympani was found, either in ears with EH or the control ears. However, the endocochlear potential, which we evaluated for position verification of the measurement pipette in the cochlea, was statistically significantly decreased in ears with EH.