Aim To investigate the protective effect of reactive oxygen species(ROS) scavenger,N-acetyl-L-cysteine(NAC),against endoplasmic reticulum stress(ERS) induced by chemical hypoxia in H9c2 cardiomyocytes.Methods H9c2 cells were treated with cobalt chloride(CoCl2),a chemical hypoxia-mimetic agent,to establish the chemical hypoxia-induced cardiomyocyte injury model.NAC was added into cell medium 60 min prior to CoCl2 exposure.The cell viability was evaluated using cell counter kit(CCK-8).Morphological changes in apoptotic cardiomyocytes were detected by Hoechst 33258 staining and photofluorography,and the intracellular ROS level was measured by 2',7'-dich-lorfluorescein-diacetate(DCFH-DA) staining and photofluorography.The expression of glucose regulated protein 78(GRP78) was evaluated by Western blot assay.Results H9c2 cell viability was inhibited by cobalt chloride at the concentrations from 100 to 2 000 μmol/L for 24 h in a dose-dependent manner.At the time from 12 h to 48 h,H9c2 cell viability was inhibited by 800 μmol/L CoCl2 in a time-dependent manner.Pretreatment with 2 000 μmol/L NAC 60 min before exposure to CoCl2 significantly inhibited not only CoCl2-induced overproduction of ROS,but also the apoptosis induced by CoCl2.GRP78 expression was upregulated after treatment with different concentrations of CoCl2 for 24 h,peaking at 800 μmol/L.Furthermore,GRP78 expression was upregulated after treatment with 800 μmol/L CoCl2 at different time(3 h,6 h,9 h,12 h,24 h),peaking at 9 h.In addition,2 000 μmol/L NAC preconditioning obviously blocked the upregulation of GRP78 expression induced by 800 μmol/L CoCl2 for 9 h.Conclusion NAC protects H9c2 cardiomyocytes against injury induced by chemical hypoxia,which may be associated with its inhibitory effect on ERS induced by chemical hypoxia.