The flavonoid morin was tested for anti-inflammatory activity in trinitrobenzenesulfonic acid (TNBS)-induced rat colitis. Rats were pretreated orally with several doses of the flavonoid (5, 10, 25, 100 and 200 mg/kg) 48, 24 and 1 h before and 24 h after colitis induction and examined for colonic damage 48 h after colitis induction. Colonic inflammation was characterized by diffuse hemorrhagic necrosis of the mucosa, bowel wall thickening, impairment of fluid absorption, increase in myeloperoxidase (MPO) activity, enhanced leukotriene B₄ (LTB₄) synthesis, glutathione depletion and increased levels of malonyldialdehyde (MDA). Morin treatment, at doses ranging from 10 to 200 mg/kg, significantly reduced colonic macroscopic damage. This beneficial effect was also confirmed by inhibition of colonic MPO activity. Several mechanisms may contribute to the protective effect exerted by morin. First, inhibition of colonic LTB₄ synthesis is a common feature for all the active doses of the flavonoid. Second, the antioxidant properties of morin, which partially prevented colonic glutathione depletion (at doses of 10 and 25 mg/kg) or inhibited colonic MDA production (at doses of 100 and 200 mg/kg), can collaborate in preventing TNBS-induced inflammation.