Inducible nitric oxide synthase (iNOS/NOS II) mediates cytotoxicity under pathological stimulation. The purpose of this study was to examine whether the blockade of NOS activity leads to a decrease in cochlear damage after intense acoustic stimulation. Guinea pigs were divided into 4 groups: (1) a noise group, (2) a NOS inhibitor (N<sup>G</sup>-nitro-L-arginine methyl ester [L-NAME]) + noise group (L-NAME/noise group), (3) an L-NAME group, and (4) a control group. Stimuli involved a pure tone at a frequency of 2 kHz for 5 hours. The sound pressure level was 120 dBSPL. In the L-NAME/noise group, 50 mg/kg body weight of L-NAME was injected 1 hour before acoustic stimulation. In the control group and the L-NAME group, acoustic stimulation was not performed. In the L-NAME group, the same dose of L-NAME was injected intraperitoneally. In the control group, only physiological saline was injected. Auditory brainstem responses (ABRs) were recorded before and immediately, 1 day, and 7 days after acoustic stimulation. The ABR threshold was significantly higher immediately after acoustic stimulation in both the noise group and the L-NAME/noise group. One day after acoustic stimulation, the threshold shift was decreased in the noise group. The threshold shift was still present 7 days after acoustic stimulation but was significantly lower in the L-NAME/noise group than in the noise group. In the L-NAME group and the control group, threshold shifts were not apparent. The lateral wall, the organ of Corti, and the spiral ganglion cells of the cochlea in both the L-NAME group and the control group did not display immunoreactivity for iNOS at any time. Immunoreactivity for iNOS was found in the lateral wall, the supporting cells (Hensen's cells, Deiters' cells, and pillar cells), and the spiral ganglion cells in both the noise group and the L-NAME/noise group. These immunoreactivities for iNOS were detected immediately, 1 day, and 7 days after acoustic stimulation. Immunoreactivity decreased over time in the stria vascularis, the organ of Corti, and the spiral ganglion cells in the noise group. The same phenomenon was observed in the L-NAME/noise group. In conclusion, iNOS was detected in cochlea damaged by acoustic stimulation. A NOS inhibitor (L-NAME) reduced the elevation of hearing thresholds. Our results suggest that the expression of iNOS participates in the pathogenesis of cochlear damage caused by acoustic trauma.