In order to investigate the effect of an earplug in experimentally-induced barotrauma, an earplug was fitted in the external ear cannal of a guinea pig. Cochlear action potentials were recorded after compression and decompression in extremely short period (2-3 seconds/1ATA-2ATA). Morphological changes were investigated by both scanning electron microscope and light microscope. A shift at high frequencies of the threshold of AP was noted in ears with an earplug. Degeneration and/or disappearance of stereocilia of outer sensory cells of the basal turn and hemorrhage in the scala tympani were observed in ears fitted with earplugs. The hemorrhage originated from veins of the scala vestibuli and collecting venules. In ears without an earplug, the cochlea was morphologically almost intact.