Overproduction of nitric oxide (NO) by inducible nitric oxide synthase (iNOS) may contribute to the pathophysiology of ulcerative colitis. A 2,4,6-trinitrobenzenesulfonic acid sodium salt (TNBS) colitis model was established to examine the effect of selective iNOS inhibition, by S-(2-aminoethyl) isothiouronium bromide (ITU), on colonic mucosal cell damage and inflammation. Rats, killed 7 days after TNBS, had increased colonic mucosal levels of iNOS and interleukin-8 (IL-8), in addition to severe colonic inflammation which was characterized by significantly increased colon weight, damage score and colonic myeloperoxidase activity (MPO) (a marker of neutrophil influx). TNBS-treated rats had markedly decreased body weight and thymus weight. Administration of colitic rats with ITU significantly inhibited iNOS activity/expression and tended to reduce mucosal levels of IL-8, but no effect on MPO activity was observed. Following ITU therapy, colitic rats had reduced colonic damage and losses in body weight and thymus weight were reversed. Improvement of TNBS colitis by ITU suggested that excess NO, produced by iNOS, may have contributed to the initiation/amplification of colonic disease, by mechanisms including enhancement of IL-8 release. NO-mediated enhancement of pro-inflammatory cytokine release was further investigated in vitro. Lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) stimulated release of nitrite, lactate dehydrogenase (LDH), TNF alpha, IL-1 beta and IL-8 from rat peritoneal macrophages, all of which were significantly reduced by ITU. This suggests that NO-mediated cell damage enhances pro-inflammatory mediator release from macrophages. In addition, enhancement of IL-8 and TNF alpha release was also partially NO-dependent in activated peritoneal neutrophils. Therefore, the amelioration of TNBS colitis by ITU could include inhibition of NO-mediated pro-inflammatory cytokine release.