Amyloid beta protein (Abeta) may be neurotoxic during the progression of Alzheimer's disease by eliciting oxidative stress. This study was designed to determine the effect of Polygonum multiflorum Thunb water extract (PWE) on Abeta25-35-induced cognitive deficits and oxidative stress in mice. Mice were fed experimental diets comprising either 0.5 or 1% PWE for 4 weeks, and then received a single intracerebroventricular (i.c.v.) injection of Abeta25-35 (10 microg/mouse). Behavioral changes in the mice were evaluated using passive avoidance and water-maze tests. The consumption of PWE significantly ameliorated the cognitive deficits caused by i.c.v. injection of Abeta25-35. The Abeta25-35 treatment accelerated the lipid peroxidation, and PWE attenuated the Abeta-induced increase in brain levels of thiobarbituric acid reactive substances. There was an increase in glutathione peroxidase activity in PWE-treated groups. The acetylcholinesterase activity in the brain and serum was lower in PWE supplemented groups than in the only Abeta-injected group. These findings suggest that PWE exerts a preventive effect against cognitive deficits induced by Abeta25-35 accumulation in Alzheimer's disease, and that this effect is mediated by the antioxidant properties of PWE.