ABSTRACT-The importance of gastrointestinal injury in endotoxin-induced shock and multiple organ failureis of great interest. In this paper we describe a method to assess the degree of intravascular congestion and bleeding into the wall of the intestine by determining the hemoglobin content of the tissue. After validating this method, we used it to study the mechanism of jejunal injury induced by intravenous injection of Escherichia coli lipopolysaccharide (LPS, 50 mg/kg bw), the role of nitric oxide release in maintaining the integrity of endothelial cells, and the participation of H202 production in the LPS-induced intestinal damage in rats. Our results show that afterthe administration of LPS at the dose of 50 mg/kg intravenously, the hemoglobin content of the jejunum (17.8 mg/100 mg tissue) increased 7.7-fold over that of control animals (2.3 mg/100 mg), reflecting a serious degree of congestion, bleeding, and damage in the gastrointestinal tract. Administration of nitro-L-arginine methyl ester (L-NAME) not only enhanced this injury, but also markedly decreased the dose of LPS necessary to induce intestinal damage. Infusion of L-arginine (300 mg/kg bolus plus infusion 600 mg/kg'h intravenously) protected the intestine against LPS or LPS plus L-NAME. Inhibition of basal nitric oxide release by L-NAME produced significant changes in cardiovascular variables, but failed to induce a significant bleeding damage. However, when inhibition of NO release was combined with enhanced H202 production by a small dose of LPS, a serious bleeding damage was observed. This was accompanied by a marked decrease in mesenteric blood flow and cardiac output. High dose of LPS induced the above effects, and thus could be responsible for the bleeding damage, while low dose of LPS that fails to inhibit nitric oxide, did not induce any intestinal bleeding. It seems that inhibition of NO release and stimulation of H202 production are both involved in the LPS-induced bleeding damage.