The present work was done to investigate the possible effects of thymoquinone on acetic acid-induced colitis in rats. Colitis was induced by intracolonic injection of 3% acetic acid. Several parameters including macroscopic score, histopathological and biochemical, were determined to assess the degree of protection. Biochemical parameters such as myeloperoxidase activity, reduced glutathione levels, platelet activating factor (PAF) and histamine were measured following standard assay procedures. The study showed that pretreatment of rats for 3 days with thymoquinone (10 mg/kg) was able to give complete protection against acetic acid-induced colitis an effect significantly higher than sulfasalazine (500 mg/kg) control group. The smaller dose of thymoquinone (5 mg/kg) produced partial protection. Moreover, the biochemical and histopathological changes were reversed and brought towards the control. These results suggest a beneficial effect of thymoquinone against experimentally-induced colitis and the possible mechanism of the protective effects may be partly due to an antioxidant action.