Ginseng was incubated under mildly acidic conditions and its inhibitory effect on a rat ischemia-reperfusion model was investigated. When ginseng was treated with 0.1% hydrochloric acid at 60 degrees C, its protopanaxadiol saponins were transformed to diasteromeric ginsenoside Rg3 and Delta20-ginsenoside Rg3. When the transformed ginseng extract, of which the main component was ginsenosides Rg3, was treated with human intestinal microflora, the main metabolite was ginsenoside Rh2. Orally administered acid-treated ginseng (AG) extract and ginsenoside Rh2 potently protect ischemia-reperfusion brain injury. The ginsenoside Rh2 also inhibited prostaglandin-E2 synthesis in lipopolysaccharide-stimulated RAW264.7 cells, but showed no in vitro antioxidant activity. These results suggest that AG and ginsenoside Rh2 can improve ischemic brain injury.