Excessive accumulation of amyloid beta-peptide (Abeta) plays an early and critical role in synapse and neuronal loss in Alzheimer's Disease (AD). Increased oxidative stress is one of the mechanisms whereby Abeta induces neuronal death. Given the lessened susceptibility to oxidative stress exhibited by mice lacking p66Shc, we investigated the role of p66Shc in Abeta toxicity. Treatment of cells and primary neuronal cultures with Abeta caused apoptotic death and induced p66Shc phosphorylation at Ser36. Ectopic expression of a dominant-negative SEK1 mutant or chemical JNK inhibition reduced Abeta-induced JNK activation and p66Shc phosphorylation (Ser36), suggesting that JNK phosphorylates p66Shc. Abeta induced the phosphorylation and hence inactivation of forkhead transcription factors in a p66Shc-dependent manner. Ectopic expression of p66ShcS36A or antioxidant treatment protected cells against Abeta-induced death and reduced forkhead phosphorylation, suggesting that p66Shc phosphorylation critically influences the redox regulation of forkhead proteins and underlies Abeta toxicity. These findings underscore the potential usefulness of JNK, p66Shc, and forkhead proteins as therapeutic targets for AD.