CONCLUSIONS Styrene exposure causes hair cell death through both apoptotic and necrotic pathways and treatment with N-acetyl-L-cysteine (L-NAC) reduces styrene ototoxicity. OBJECTIVE Exposure to styrene causes hearing loss and hair cell death in the middle frequency region in the cochlea. The current study was designed to examine the cell death pathways and the protective effect of L-NAC against styrene-induced cochlear injuries. METHODS Seventeen rats were exposed to styrene by gavage at 400 mg/kg 5 days per week for 3 weeks. Nine of the styrene-treated rats received L-NAC by intraperitoneal injection (325 mg/kg), and the remaining eight rats received saline injections as controls. The styrene-induced hearing loss was assessed by auditory brainstem responses (ABRs). Apoptotic, necrotic, and missing hair cells were quantified using combined methods, including nuclear staining with propidium iodide, F-actin staining with FITC-phalloidin, and the TUNEL assay. RESULTS The styrene exposure caused a threshold shift of 15±4.3 dB. Both apoptosis and necrosis were involved in the pathogenesis of the cochlear lesion, but apoptosis appeared to be the major cell death pathway leading to the styrene ototoxicity. Treatment with L-NAC reduced the number of missing and dying outer hair cells (OHCs) and reduced the styrene-induced hearing loss.