First reports on ototoxicity of medicines were published by Kirchner (1883) and Wittmaack (1903). Gentamicin is the most frequently used aminoglycoside of the second generation. Experimental doses of gentamicin reach even 200 mg/kg, and their ototoxicity is proven. The aim of work was an estimation of the tuner ear sensory cells after administration of minimal doses of the antibiotic. The control group consisted of 10 guinea pigs, which were given 2 ml/kg of 0.9 % NaCl. The experimental group consisted also of 10 animals. They were given gentamicin 25 mg/kg i.m., once a day for 10 consecutive days. After completion of the experiment, cochlear microphonics with modified phase-sensitive detection technique were received. They were done for 6 frequencies from 260 Hz to 8 kHz, and 4 levels of acoustic impulse pressure from 55 to 95 dB. Next, the cochleae were prepared for examination in a scanning electron microscopy. Significant differences between the cochlear microphonics in both groups were not found. In all animals correct pictures of sensory cells were also observed, but in the experimental animals disturbances of microphonics phase for the frequencies 2, 4, and 8 kHz were detected. This may be indicative of cochlear frequency position changes in the submicroscopically injured inner ear.