Perilymph fistula has been known to be a cause of sudden hearing loss. The fistula was experimentally created in the round window of guinea pigs. The threshold and latency of cochlear nerve action potential were observed at the frequencies of 2KHz, 4KHz and 6KHz before and after creating the round window fistula. These parameters were recorded from the external auditory canal, using needle electrodes. The results were as follows : 1) The thresholds of the cochlear nerve action potential were 42.0±18.8 dB SPL at 2KHz, 39.0±18.2 dB SPL at 4KHz, and 28.6±16.8 dB SPL at 6KHz 2) The shifts of the threshold were 23.0±19.0 dB at 2KHz, 22.7±26.2 dB at 4KHz, and 28.2±29.4 dB at 6KHz, and the overall shift was 24.4 dB. 3) After the creation of the round window fistula, the latency was delayed by 0.17 msec at 90 dB SPL on the average.