OBJECTIVE: Ulcerative colitis is a chronic recurrent inflammatory bowel disease of unknown etiology. The anti-inflammatory, immunomodulatory, and antioxidant characteristics of Henna ( Lawsonia inermis) fixed oil (HFO) imply that it may be advantageous for the treatment of colitis. MATERIALS AND METHODS: In this research, the effect of HFO in a Wistar albino rat model of acetic acid (AA)-induced ulcerative colitis, was examined. The animals received daily oral administration of either normal saline (10 ml/kg), HFO (100, 400, and 1600 microl/kg), or dexamethasone (2 mg/kg) for 5 days. A single intracolonic injection of 2 ml of a 4% (v/v) acetic acid solution was used to induce colitis. The levels of myeloperoxidase (MPO) and tumor necrosis factor-alpha (TNF-alpha) were measured. RESULTS: The administration of HFO at doses 400 and 1600 mul/kg showed a significant enhancement in the weight-to-length ratio of colon tissue in comparison to the control group. Furthermore, the increased amounts of HFO (400 and 1600 mul/kg) were associated with a significant reduction in ulcer severity, area, and index. However, examination of tissue samples revealed a decrease in the overall colitis index suggesting fewer inflammatory cells invaded the colonic regions of rats treated with HFO at doses of 400 and 1600 mul/kg. Moreover, the elevated MPO levels and TNF-alpha were significantly decreased following the administration of the fixed oil at these doses. CONCLUSION: These findings indicate that HFO could potentially decrease the manifestations of experimental colitis in a dose-dependent manner.