We studied the mechanism of the superoxide generation system in indomethacin-induced gastric mucosal injury. First, 10 mM indomethacin had no direct effect on xanthine oxidase (XOD) activity. Next, we found that NADPH oxidase activity in polymorphonuclear leukocytes (PMN) of peripheral blood was significantly increased 6 h after administration of indomethacin. This phenomenon was inhibited by the injection of the NADPH oxidase inhibitor, diphenylene iodonium chloride (DIC). Activation of NADPH oxidase caused the component, p47phox to be translocated to the plasma membrane. Since indomethacin did not directly activate NADPH oxidase, we sought another route of activation of PMN. As IL-1 and TNF alpha play in the inflammation, we examined these cytokines in this study. TNF alpha was not detected but IL-1 was increased significantly 30 min after administration of indomethacin.