Abstract: Acoustic impedance changes and hearing threshold shifts after exposure to impulse noise or air-blasts were studied in guinea pigs. Air-blasts were produced by firing of live ammunition in a 100 mm caliber gun, the peak SPL ranging from 172.9 to 190.3 dB for different groups of animals. Each exposure consisted of 7-10 impulses. Blast impulses of the same origin were recorded on a tape recorder and reproduced by playing back through loudspeaker system to serve as the source of impulse noise. Each exposure consisted of 400 impulses, the peak SPL being 160 dB. Changes in tympanograms of most animals after exposure to air-blasts suggested that hearing threshold shifts in this group of animals were probably due to impairment of the middle ear. With respect to impulse noise, there was no evidance of severe damage to the middle ear althouth threshold shifts were obvious. It was proposed that inner ear trauma might be responsible for the latter changes. Cases with normal and abnormal tympanograms in the performed experiment were also discussed.