Maintainence of appropriate redox homeostasis is crucial for processes such as protein folding in the endoplasmic reticulum (ER) and to minimise genesis of oxidative stress. Previous studies have indicated a possible link between ER stress and production of reactive oxygen species (ROS) although the cellular mechanisms involved were not fully elucidated. To investigate the cellular mechanisms involved in tolerance to oxidative stress and ER stress, genome-wide screens were performed to identify mutants sensitive to chronic ER stress induced by dithiothreitol and tunicamycin. These screens identified the Cu,Zn superoxide dismutase (SOD1) and genes involved in NADPH generation (RPE1, TKL1) as important for chronic ER stress tolerance. Superoxide anion has been identified as one of the ROS generated during ER stress. The ER oxidoreductase Ero1p, previously implicated in ROS production in vitro, did not appear to be a source of superoxide when the protein was over-expressed. It was also found that cellular NADP(H) levels affected induction of the unfolded protein response (UPR), since cells lacking TKL1 or RPE1 exhibited decreased UPR induction. These data indicate an important role for superoxide dismutase and cellular NADP(H) in survival of cells during ER stress. Subsequent analysis determined that NADPH generation was also required for adaptation to H2O2. Mutants affected in NADPH production were chronically sensitive to H2O2 but resistant to an acute dose. These mutants over-accumulated reduced glutathione (GSH) but maintained normal cellular redox homeostasis. This over- production of GSH was not regulated at the transcriptional level of GSH1 encoding ??- glutamyl cysteine synthetase. These data raise the important question as to how cells maintain cellular glutathione redox balance. To better understand how cells respond to perturbations in glutathione redox homeostasis, cells deleted for GLR1, encoding GSSG reductase, were exposed to extracellular oxidised glutathione (GSSG) and intracellular GSH and GSSG were monitored over time. Intriguingly cells lacking GLR1 showed increased levels of GSH accumulation upon GSSG treatment in a manner independent of GSH synthesis. It was subsequently found that the cytosolic thioredoxin-thioredoxin reductase system contributes to the reduction of GSSG in vivo.