Background: The role of NO in inflammatory bowel disease is controversial. Studies indicate that endothelial nitric oxide synthase (eNOS) might be involved in protecting the mucosa against colonic inflammation. The aim of this study was to investigate the involvement of NO in regulating colonic mucosal blood flow in two different colitis models in rats. Methods: In anesthetized control and colitic rats, the distal colon was exteriorized, and the mucosa visualized. Blood flow (laser-Doppler flowmetry) and arterial blood pressure were continuously monitored throughout the experiments and vascular resistance was calculated. Trinitrobenzenesulfonic acid (TNBS) or dextran sulfate sodium (DSS) was used to induce colitis. All groups were given the NOS inhibitor L-NNA or the iNOS inhibitor L-NIL. iNOS, eNOS and nNOS mRNA in colonic samples was investigated with real time RT-PCR. Results: Before NOS inhibition, colonic mucosal blood flow, expressed as perfusion units, was higher in both colitis models compared to the controls. The blood flow was reduced in the TNBS- and DSS-treated rats during L-NNA administration, but was not altered in the control group. Vascular resistance increased more in the TNBS- and DSS-treated rats than in the control rats, indicating a higher level of vasodilating NO in the colitis models. L-NIL did not alter blood pressure or blood flow in any of the groups. iNOS and eNOS mRNA increased in both colitis models while nNOS remained at the control level. Conclusion: TNBS and DSS induced colitis results in increased colonic mucosal blood flow, most probably due to increased eNOS activity.