Historically, occupational hearing impairment due to noise has been known on blacksmithes from the Bronze Age. Moreover, after The Industrial Revolution, patients sufferring from noise induced hearing loss has been increased because of mechanization of factories and firearms, development of transportation such as automobile and aircraft etc. Though there has been marked development of audiology, histopathology and biochemistry of the inner ear, many of the mystery of the noise induced hearing damages are not clarified. Fowler (1928) found 4,000 Hz dip phenomena in noise induced hearing loss and cochlear damages in noise trauma were observed histopathologically by Hamberger and Hyden (1945) with guinea pigs. Recently Ades and Stockwell (1969) reported relationship between regional suceptibility and sound frequencies. They found the hair cells of the basal turn were damaged due to high frequencies, and apical turn due to low frequencies. Now in this study, we are going to confirm the cochlear hair cell damages in guinea pigs due to exposure of 4,000 Hz pure tone, 1,000 Hz pure tone, white noise and jazz music sound with intensity of 112~114 dB S.P.L. respectively for 4 hours continuousely. In observation of the hair cells, we use the phase contrast microscope by means of the surface preparation technique.