Using a novel experimental model of chronic enterocolitis described by Morris et al., we observed sequential changes of mucosal lesions endoscopically and performed histopathological studies. Fisher rats were rectally administered 25 mg of trinitrobenzene sulfonic acid (TNBS) dissolved in 0.5 ml of 50% ethanol (ET). The combination treatment of TNBS and ET produced colitis in rats for over 3 weeks. TNBS itself did not induce any lesions. ET alone induced mucosal lesions, but their severity was much smaller than that induced by TNBS/ET. As an animal model much closer to human inflammatory bowel disease, we have newly developed a canine model of chronic ileitis. Adult mongrel dogs were administered 10 ml of 100% ethanol and 1 g of TNBS dissolved in 10 ml of distilled water (i.e., 100 mg/ml solution) through a 4-lumen double balloon tube which was inserted into the ileum. The TNBS/ET-induced ileitis in dogs persisted for 8 weeks. The mucosal lesions induced by TNBS/ET were characterized as annular or longitudinal ulcers accompanied by extensive lymphocyte infiltration and granulomas, which were similar to macro- and microscopic findings observed in human Crohn's disease. Endoscopic examinations were a valuable tool to obtain sequential information on the development of inflammatory changes in each individual animal. Our canine model would provide various advantages for the study of functional impairment in chronic enterocolitis as well as for the detection of potential therapeutic agents in the human counterpart.