Abstract： Objective To investigate the effects of N - acetylcysteine (NAC) on apoptosis and the expressions of Fas/FasL mRNA in lung tissue of rats with paraquat - induced acute lung injury.Methods Forty five male SD rats were randomly (random number) divided into normal control group,paraquat (PQ) group,and NAC treatment group.The rat model of acute lung injury was made with 2％ PQ induction in dose of 25 mg/kg injected,and NAC was injected into the PQ poisoning rats (200 mg/kg) 30 minutes after PQ administration in NAC treatment group.In the control group,equal amount of saline instead was injected into the rats.Apoptosis was detected by using TUNEL method and the expressions of Fas/FasL mRNA were evaluated by using reverse transcription polymerase chain reaction (RT- PCR),and the levels of Fas/FasL protein were detected by using western blot analysis.Results Compared with control group,cell apoptosis and expressions Fas/FasL mRNA in PQ group were significantly different ( P ＜ 0.05 ).Compared with PQ group,cell apoptosis and expressions Fas/FasL mRNA in NAC group were significantly decreased,were significant lower (P ＜ 0.05).Conclusions NAC inhibited apoptosis in lung tissue of rats with paraquat induction by regulating the activation of Fas/FasL systems.