The effect of loud sound on the endolymphatic dc potential was studied in both normal and hypoxic animals. When the integrity of the cochlear wall was maintained, the endolymphatic dc potential decreased 5 mv with the onset of sound (1000 cps below 140 db) and recovered immediately when the sound was terminated. At intensities of 140 db and above, the dc potential irreversibly decreased. The effect of sound on the dc potential was the same for both normal and hypoxic animals. A negative dc potential was found in the scala media of hypoxic and dead animals. This potential survived the life span of the post‐mortem microphonics. The positive dc potential returned on termination of hypoxia. Oxygen deprivation resulted in the dc potential decreasing before the microphonics. The possible significance of these findings to the physiology of hearing is discussed.