Most of sensorineural hearing loss is caused by cochlear hair cell injury. Evidence for involvement of cochlear inflammation in hair cell damage came from the finding that activation of macrophages causes hair cell loss. In this study, we sought to determine the involvement of macrophages in the onset of hearing loss. The mice were exposed to noise at a 90-dB sound pressure level for 1 h per day for 5 days and measured the auditory threshold at the frequencies of 12, and 20 kHz.  Cochlear macrophages were visualized by immunostaining used anti-F4/80 (macrophage marker) and anti-CD11b (microglia marker).  Minocycline (microglia activation inhibitor) was intraperitoneally administered at a dose of 50 mg/kg once a day during noise exposure. The auditory threshold markedly increased at the frequencies of 12 and 20 kHz in the noise-exposure time-dependent manner. Noise exposure significantly increased macrophages positive to both F4/80 and CD11b closely localized (50 μm) to the inner hair cells. Treatment with minocycline significantly prevented the noise-induced elevation of the auditory threshold. Thease data suggest that activation of cochlear macrophages is involved in the onset of sensorineural hearing loss.