Abstract: Objective: To explore the influence of nitric oxide in serum on aminoglycoside antibiotic induced deafness in guinea pigs and its mechanism.Methods: Twenty four health guinea pigs were randomly divided into four groups:6 animals acted as normal controls,6 were given gentamyin,6 gentamyin with L-arginine from 1 to 19 days and 6 gentamycin with L-arginine from 10 to 19 days.Then the nitric oxide in serum was measured.The threshold of auditory brainstem response(ABR) and the pathological morphology of outer hair cells in the inner ear were evaluated.Results: The nitric oxide in serum of the gentamycin group was not significantly higher than that of the normal control group(P>0.05),the ABR threshold(95.63 dB) was significantly higher than that of the normal control group(P<0.05),and the outer hair cells were massively lost.The nitric oxide in serum in the L-arginine groups was much higher than that in the normal control group and that in the gentamycin groups(P<0.001-P<0.005),but the ABR threshold was lower than that in the gentamycin group and the outer hair cells were sporadically lost.Conclusions: Some degree of nitric oxide in serum may cause toxicity to outer hair cells in guinea pigs,but higher level of nitric oxide in serum may be effective in prevention of outer hair cells in guinea pigs.The L-arginine may decrease ABR threshold and alleviate outer hair cells injury,which has preventive effect to the cochlea of guinea pigs.