Background & Aims Hydrogen sulfide (H2 S) is an endogenous gaseous mediator of mucosal defense with antiinflammatory effects that promote ulcer healing. The effects of H2 S during the pathogenesis of colitis have not been established. We analyzed the contribution of H2 S to inflammation and ulceration of the colon in a rat model of colitis. Methods Colitis was induced by intracolonic administration of trinitrobenzene sulfonic acid. The ability of the colon to synthesize H2 S was studied over the course of the resolution of the colitis. Expression of 2 enzymes involved in the synthesis of H2 S and the effects of inhibitors of these enzymes were examined. We also examined the effects of H2 S donors on the resolution of colitis. Results The capacity for the colon to produce H2 S increased markedly over the first days after induction of colitis and then declined toward control levels as the colitis was resolved. Inhibition of colonic H2 S synthesis markedly exacerbated the colitis, resulting in significant mortality. Inhibition of H2 S synthesis in healthy rats resulted in inflammation and mucosal injury in the small intestine and colon along with down-regulation of cyclooxygenase-2 messenger RNA expression and prostaglandin synthesis. Intracolonic administration of H2 S donors significantly reduced the severity of colitis and reduced colonic expression of messenger RNA for the proinflammatory cytokine tumor necrosis factor α. Conclusions In rats, H2 S modulates physiological inflammation and contributes to the resolution of colitis.