Sudden hearing loss is an otologic emergency. Until recently, the etiopathologic cause is not verified. The purpose of this experiment is to investigate the relationship between round window fistula and sudden hearing loss. Round window was exposed through the superior approach, and was perforated with a micropipette. Electrophysioloical examination of the auditory function was carried out three times, i.e., before the membrane injury, after the membrane injury, after closure of the round window fistula. The results were as follows ; 1) In ten guinea pigs, we observed that the detection threshold of the wave V was 10dB HL. 2) After making the round window fistula, the amount of hearing loss was 10dB HL. 3) After closure of the round window fistula with soft tissue and Gelfoam, there was some degree of hearing gain. The amount of hearing gain was 5dB HL.