Quercitrin was tested for acute and chronic anti-inflammatory activity in trinitrobenzenesulfonic acid-induced rat colitis. The inflammatory status was evaluated by myeloperoxidase, alkaline phosphatase and total glutathione levels, leukotriene B4 synthesis, in vivo colonic fluid absorption, macroscopical damage and occurrence of diarrhea and adhesions. Treatment with 1 or 5 mg/kg of quercitrin by the oral route reduced myeloperoxidase and alkaline phosphatase levels, preserved normal fluid absorption, counteracted glutathione depletion and ameliorated colonic damage at 2 days. Increasing or lowering the dose of the flavonoid resulted in marked loss of effect. The acute anti-inflammatory effect of quercitrin is unrelated to impairment of neutrophil function or lipoxygenase inhibition, and it may be caused by mucosal protection or enhancement of mucosal repair secondary to increased defense against oxidative insult and/or preservation of normal colonic absorptive function. When tested in chronic colitis (2 and 4 weeks), quercitrin treatment (1 or 5 mg/kg.day) decreased colonic damage score and the incidence of diarrhea, and normalized the colonic fluid transport. All other parameters were unaffected. The chronic effect of the flavonoid is apparently related to its action on colonic absorption, although it can be partly secondary to its acute beneficial effect.