Amyloid β-peptide (Aβ), the major constituent of the senile plaques in the brains of patients with Alzheimer's disease, is cytotoxic to neurons and has a central role in the pathogenesis of the disease. Previous studies have suggested that oxidative stress is involved in the mechanisms of Aβ-induced neurotoxicity in vitro. In the present study, we examined whether oxidative stress contributes to learning and memory deficits caused by continuous intracerebroventricular infusion of Aβ-(1–42). In the Aβ-(1–42)-infused rats, spontaneous alternation behaviour in a Y-maze and spatial memory in a water maze task were significantly impaired, as compared with Aβ-(40–1)-infused control rats. The retention of passive avoidance learning was also significantly impaired by treatment with Aβ-(1–42). Potent antioxidants idebenone and α-tocopherol prevented the behavioural deficits in Y-maze and water maze, but not passive avoidance, tasks in Aβ-(1–42)-infused rats when they were repeatedly administered by mouth once a day from 3 days before the start of Aβ infusion to the end of behavioural experiments. Lipid peroxide levels in the hippocampus and cerebral cortex of Aβ-(1–42)-infused rats did not differ from those in control animals, and neither idebenone nor α-tocopherol affected the lipid peroxide levels. These results suggest that treatment with antioxidants such as idebenone and α-tocopherol prevents learning and memory deficits caused by Aβ.