The threshold shifts in AP after exposure to an intense pure tone were investigated in hydropic guinea pigs. The endolymphatic duct and sac were obliterated to induce endolympatic hydrops experimentally in 32 albino guinea pigs. Electrophysiological recording and acoustic overstimulation were performed in the 6th, 12th or 24th postoperative week. A pair of AP thresholds was measured at each frequency from 2 to 16kHz before and after acoustic overstimulation (2kHz, 110 or 120dB SPL, 30minutes), and the threshold shift caused by acoustic overstimulation was determined at each frequency. The following results were obtained : 1) threshold shifts in hydropic guinea pigs were smaller than those reproted by us in 1990 in normal guines pigs ; 2) the frequency at the maximal threshold shift was much higher in hydropic guinea pigs than in normal ones. Our results can be explained on the basis of cochlear hydrodynamics ; namely, the movement of the ba-silar membrne is damped earlier, and the maximal amplitude point of the travelling wave is shifted toward the stapes in hydropic cochlea. This change in the hydrodynamics of the cochlea may explain the mechanism of the development of diplacusis in Ménière's disease.
