Clinical studies suggest a relationship between folate deficiency and neurological and disorders including Alzheimer's disease (AD). To investigate mechanisms underlying this association, we examined the consequences of folate deprivation on neuronal cultures. Culturing embryonic cortical neurons and differentiated SH-SY-5Y human neuroblastoma cells in folate-free medium induced neurodegenerative changes characteristic of those observed in AD, including increased cytosolic calcium, reactive oxygen species (ROS), phospho-tau and apoptosis. In accord with clinical studies, generation of the neurotoxic amino acid homocysteine (HC) was likely to contribute to these phenomena, since (1) a significant increase in HC was detected following folate deprivation, (2) addition of the inhibitor of HC formation, 3-deazaadenosine, both prevented HC formation and eliminated the increase in ROS that normally accompanied folate deprivation, (3) direct addition of HC in the presence of folate induced the neurotoxic effects that accompanied folate deprivation, and (4) an antagonist of NMDA channels that blocks HC-induced calcium influx also blocked calcium influx following folate deprivation. Folate deprivation decreased the reduced form of glutathione, indicating a depletion of oxidative buffering capacity. This line of reasoning was supported by an increase in glutathione and reduction in ROS following supplementation of folate-deprived cultures with the cell-permeant glutathione precursor, N-acetyl-L-cysteine, or vitamin E. Folate deprivation potentiated ROS and apoptosis induced by amyloid-beta, while folate supplementation at higher concentrations prevented generation of ROS by amyloid-beta, suggesting that folate levels modulate the extent of amyloid-beta neurotoxicity. These findings underscore the importance of folate metabolism in neuronal homeostasis and suggest that folate deficiency may augment AD neuropathology by increasing ROS and excitotoxicity via HC generation.