N‐acetyl cysteine (NAC), a metabolite of sulphur‐containing amino acid cysteine, is used as an antioxidant and a mucolytic agent. Therefore, we aimed to investigate anti‐inflammatory and anti‐ulcerative effects of NAC. We also intended to determine the relation between antiulcer effect of NAC and its antioxidant properties by biochemical evaluation. In this study a total of 15 rat groups (n = 6 per group) were used for inflammation and ulcer experiments. Anti‐inflammatory effects of NAC have been investigated on six rat groups with carrageenan (CAR)‐induced paw oedema model. Antiulcer effects of NAC have been investigated on 24 h fasted nine rat groups with IND‐induced ulcer model in the presence of positive (LAN, RAN, FAM, and OMEP), negative (untreated IND group) and intact control groups. In biochemical analyses of stomach tissues; glutathione S‐transferase (GST), catalase (CAT), myeloperoxidase (MPO), and superoxide dismutase (SOD) enzyme activities and lipid peroxidation (LPO) and the glutathione (GSH) levels were determined. All doses of NAC exerted significant anti‐inflammatory effect; even the effect of 900 mg/kg NAC was similar with that of DIC and IND. In gastric tissues NAC administration decreased the level of LPO and activity of CAT, which were increased by IND. Furthermore, NAC increased the GSH level and SOD and GST activities, which decreased in ulcerous stomach tissues. Only MPO activity increased in both IND and NAC groups when compared to healthy rat group. We determined that NAC has both anti‐inflammatory and anti‐ulcerative effects. J. Cell. Biochem. 117: 308–319, 2016. © 2015 Wiley Periodicals, Inc.