lsoflavonoids, formononetin, 9,10-dimethoxypterocarpan 3-O--d-glucoside, ononin, calycosin 7-O-glc and calycosin, were isolated fromthe roots of Astragalus mongholicus Bunge (Leguminosae). The neuroprotective roles and direct antioxidant effects of these isoflavonoids wereinvestigated by using PC12 cell model and DPPH (1,1-diphenyl-2-picrylhydrazyl) assay. Formononetin, ononin and calycosin were foundinhibiting glutamate-induced cell injury, with an estimated 50% effective concentration (EC50) of 0.027 g/ml, 0.047 g/ml and 0.031 g/ml,respectively. Pretreatment with them increased the activities of antioxidant enzymes, including superoxide dismutase (SOD) and glutathioneperoxidase (GSH-Px), and prevented the release of lactate dehydrogenase (LDH) in glutamate-injured PC12 cells. On the other hand, calycosin7-O-glc and calycosin showed more scavenging activity to DPPH radicals than formononetin in the cell-free system. The inconsistency betweenthe neuroprotective capabilities of isoflavonoids and their directly scavenging activity to DPPH radicals indicated that formononetin, ononinand calycosin probably depended on increasing endogenous antioxidant and stabilizing the cells’ membrane structures to inhibit the celldamage induced by glutamate.