Increasing evidence supports the role of reactive oxygen species (ROS) in the pathogenesis of Alzheimer's disease (AD). Both in vivo and in vitro studies demonstrate that thioredoxin (Trx) and thioredoxin reductase (TR), the enzyme responsible for reduction of oxidized Trx, have protective roles against cytotoxicity mediated by the generation of ROS. The present study measured levels of Trx protein and activities of TR in the brain in AD compared with control subjects, and evaluated the possible protective role of TR and Trx against amyloid beta-peptide (Abeta) toxicity in neuronal cultures. Analysis of Trx protein levels in 10 AD and 10 control subjects demonstrated a general decrease in all AD brain regions studied, with statistically significant decreases in the amygdala (p <.05), hippocampus/parahippocampal gyrus (p <.05), and marginally significant (p <.10) depletions in the superior and middle temporal gryi. Thioredoxin reductase activity levels were increased in all AD brain regions studied with statistically significant increases occurring in AD amygdala (p =.01) and cerebellum (p =.007). To investigate the protective effects of Trx and TR against Abeta-induced toxicity, primary hippocampal cultures were treated with Trx or TR in combination with toxic doses of Abeta. Treatment of cultures with Trx led to a statistically significant concentration-dependent enhancement in cell survival against Abeta-mediated toxicity as did treatment with TR. Together, these data suggest that, although TR is protective against Abeta-mediated toxicity, the increase observed in AD brain offers no protection due to the significant decrease in Trx levels. This decrease in the antioxidant Trx-TR system may contribute to the increased oxidative stress and subsequent neurodegeneration observed in the brain in AD.