The effects of noise exposure were studied in fifty-nine 4- to 5-week-old albino Hartley guinea pigs with normal hearing (body weight 250-300 g). The follow-ing experiments and results were carried out: exposure to 350 Hz pure tones at 115 dB for 40 h and exposure to 350 Hz tones at 120 dB for 64 h. In order to investigate the effects of low-frequency tone exposures on the hearing of the guinea pigs, cochlear microphonics (CM), whole nerve action Potentials (AP) and endocochlear potentials (EP) were measured. With a high-sound pressure, a decrease was observed in the CM maximum Output voltage in the test fre-quencies of 2,4 and 6 kHz while the CM threshold (pseudothreshold) of 6 kHz was elevated. Output voltage of the Ni potential of the AP using a 7-kHz tone burst decreased while the threshold of the Ni potential was elevated. An exten-sion of latency and a decrease in the absolute value of the negative potential in EP were also observed.