The use of oral and intravenous cyclosporin represents a significant advance in the therapy of refractory inflammatory bowel diseases (IBD). However, oral administration of cyclosporin is fraught with improper delivery of cyclosporin to the colon for its topical action. Because of unpredictable metabolism by cytochrome P-450 IIIA, the targeted blood level for systemic effect is not reached at low doses. Furthermore, the doses that have been used for therapy of IBD have been shown to induce several adverse side effects. Thus, an alternate method of delivering cyclosporin to the colon is desirable. In this study, the effect of intracolonically administered cyclosporin was tested for its efficacy to heal mucosal erosions in dextran sulfate sodium (DSS)-induced colitis in mice. Both acute and chronic colitis was induced by feeding female Swiss-Webster mice with 5% DSS (30,000-40,000 mol wt) for five or seven days, respectively. Therapy was advocated prophylactically, prophylaxis plus therapy and therapeutically during the acute and chronic phase of the disease and therapeutically during the chronic phase of the disease. Intracolonic cyclosporin given prophylactically showed adverse effects by increasing the damage to the colonic mucosa. However, intracolonic cyclosporin given therapeutically in 2.5, 5, and 10 mg/kg after the induction of colitis resulted in dramatic responses in terms of reducing the disease activity and histologic scores, corroborated by complete histological resolution compared to oral cyclosporin given at identical doses. Intracolonic cyclosporin (5 mg/kg) was also very effective in reducing the chronic inflammation. The results of this study highlight the application of this animal model for therapeutic research. Furthermore, cyclosporin administered as an enema provides a new stratagem for the therapy of IBD because of its rapid onset of action at very low doses without the risk inherent in oral or systemic administration.