The purpose of this study is to investigate whether repeated noise exposure aggravates the level of permanent noise-induced hearing loss (NIHL) in diabetic rats and whether N-acetylcysteine (NAC), a precursor of glutathione, attenuates the level of noise-induced permanent hearing loss in diabetic rats. Fifty male Wistar rats were divided into four groups: 12 non-diabetic control rats with saline injection (Control-Saline), 11 non-diabetic control rats with NAC injection (Control-NAC), 13 streptozotocin-induced diabetic rats with saline injection (Diabetes-Saline) and 14 streptozotocin-induced diabetic rats with NAC injection (Diabetes-NAC). NAC (325mg/kg) was given by intraperitoneal injection twice per day (b.i.d.) for 14 days starting 2 days before noise exposure. All rats were exposed to noise for 8hours per day for 10 consecutive days to develop noise-induced permanent hearing loss. The hearing status of all animals was evaluated with auditory brainstem responses (ABR) evoked by clicks and tone bursts. ABRs were measured before and at 1hour, 1 week, 2 weeks and 4 weeks after noise exposure. After a recovery time of 4 weeks, animals were decapitated, and the loss of hair cells was assessed microscopically. In all groups, ABR thresholds failed to return to pre-exposure values throughout the experimental period. The ABR threshold to clicks was markedly elevated in the Diabetes-Saline group (36.9+/-2.3dB SPL), less elevated in the Control-Saline and Diabetes-NAC groups and least in the Control-NAC group (19.5+/-2.0dB SPL) at 4 weeks after noise exposure. Diabetes caused increased susceptibility to noise-induced hearing loss, and NAC treatment reduced the loss in both control and diabetic rats. Cochleograms revealed no gross destruction of hair cells in the non-diabetic groups or the Diabetes-NAC group; however, a significant number of outer hair cells (OHCs) were lost in the Diabetes-Saline group. This study demonstrated that diabetics were prone to developing more severe NIHL than non-diabetics and that NAC could preserve most OHCs and attenuate the permanent noise-induced hearing loss in both groups.