Two series of animal experiments were carried out to clarify the combined effect of gentamicin (GM) and noise on the auditory brainstem response. In the first experiment, 64 rats were divided into eight groups: six groups for GM treatment (0, 20, 30, 40, 50 and 100 mg/kg, respectively), one group for exposure to 110 dB SPL white noise and one for control. After four weeks of injections, no prolonged latency of peak 1 was found in the rats injected with less than 30 mg/kg GM. Prolongation was observed in rats injected with 50 mg/kg GM. When the rats were exposed to the noise for three hours a day in the third week, the latency of peak 1 was temporarily prolonged in response to a four-kHz tone burst. The prolonged latency of peak 1 returned gradually to the initial level. In the second experiment, the obvious and irreversible prolonged latencies in response to four-and 16-kHz tone bursts were found in the rats given 50 or 30 mg/kg GM for four weeks, combined with a one-week exposure to 110 dB SPL white noise. The combined effects of GM and noise typically appeared in the latency of peak 1 in response to four kHz and then 16 kHz. The combined effects were synergistic with a GM effect and a noise effect. The results suggest that hearing damage is easily caused by noise during GM therapy.