Four divers were investigated simulating a diving of 350m with saturated heliox under compression, performed in Jan 1989 at Navy Medical Institute. Changes in hearing threshold, brainstem evoked response and acoustic impedance were observed. The 4 divers experienced no tinnitus, hard of hearing and earache during compression and decompression. Objective examination showed loss of lower frequency range of hearing, that was due to the masking effect of the noise in the chamber. Changes in the wave--form and latency of brainstem evoked response were due to a change in sound wave transmission effected by the chamber pressure and a poor signal to noise ratio in the hyperbaric chamber of helium oxygen environment. All the changes were transient, after leaving the chamber, the hearing threshold and brainstem evoked response all returned to normal. Besides, there were no changes in the tympanogram, acoustic compliance and stapedius reflex before and after the experiment. Therefore the profile of compression and decompression in the experiment brought no harm to the divers' acoustic system; their Eustachian tubes, middle and inner ears functioned normally.