Hyperbaric chamber dives at 31 ATA with helium-oxygen were performed at the Japan Marine Science and Technology Center in 1987. During simulated underwater experiments, auditory brain stem responses were recorded in 4 professional divers for assessment of brain stem function. All divers had no clinical symptoms at 150 m below sea level, and their ABRs also showed no significant changes. During the 150-250 m depth saturation dives, all divers complained of various symptoms such as euphoria, ataxia, joint pain, tremor and dyspnea, while, I-III and I-V interpeak latencies on their ABRs increased with a tendency of recovery. Furthermore, the changes of both interpeak latency were independent of each other. These results indicate that transient dysfunction clinically or subclinically occurred at the processes between 150-300 m below sea level. Moreover, independent changes of I-III and I-V interpeak latencies in this study may mean that the pathways reached to the generation sites of wave III and V were different.