Summary The in vitro antioxidative activity of neopterin and its in vivo potency to suppress gastric ischemia were investigated. The scavenging activity of the reduced form of neopterin, 5,6,7,8-tetrahydroneopterin (NPH4), against superoxide anion radicals, showed an extremely high superoxide anion radical scavenging activity in two assay systems, i.e., hypoxanthine/xanthine oxidase (HPX/XOD) and rat macrophage/ phorbol myristate acetate (Mø/PMA) radical-generating systems. The antioxidative activities against both super oxideanion and hydroxyl radicals were confirmed by spin trapping-ESR spectrometry. In contrast, the oxidized form of neopterin (NP) did not show any activity in the HPX/XOD system, but was effective in the Mø/PMA system. These data suggest that NPH4 directly scavenges superoxide anion radicals, while NP suppresses the superoxide anion generation in macrophages through its inhibitory effect on the membrane NADPH-oxidase. Furthermore, NPH4 and NP effectively suppressed the formation of gastric injury induced by ischemia and reperfusion. These results suggest that neopterin play an important role as an endogenous antioxidant.