Neutrophil-mediated oxidative stress on the rat mesenteric microcirculation was studied in the experimental model of endotoxin-induced disseminated intravascular coagulation (DIC) by using an intravital fluorescent technique and luminol-dependent chemiluminescence (ChL) analysis. Leukocytes sticking to the venules were visualized by the injection of acridine orange, a fluorochrome tracer which shows high affinity to white cells. Endotoxin (E coli, O-111B4, Difco, USA) was infused intravenously at a dose of 2 mg/kg/hr. After starting the infusion of endotoxin, the number of sticking cells were gradually increased on the venular endothelium followed by a transient neutropenia. In order to investigate the distribution of infused endotoxin in the microvasculature, FITC-labeled endotoxin (Sigma, USA) was used. After administration of FITC-endotoxin, multiple patches of fluorescence along the venular walls were observed, while no fluorescent conjugates were found at the sticking neutrophils and along the arteriolar walls. ChL activities of neutrophils were also dramatically elevated, which may reflect the enhanced ability to generate oxyradical species. To investigate the inhibitory effects of heparin sodium and gabexate mesilate which was a synthetic protease inhibitor on locomotive and metabolic changes of neutrophils induced by endotoxemia, both agents were administered prior to endotoxin infusion. Gabexate mesilate attenuated these changes, but heparin sodium did not show any improving effects. It was concluded that endotoxin primarily affects the venular endothelial cells, resulting in the activation of neutrophils. Gabexate mesilate was more likely to attenuate neutrophil-mediated oxidative stress on microvasculature in endotoxin-induced DIC than heparin sodium.