Thioredoxins are small, highly conserved oxidoreductases that are required to maintain the redox homeostasis of the cell. They have been best characterized for their role as antioxidants in protection against reactive oxygen species. We show here that thioredoxins (TRX1, TRX2) and thioredoxin reductase (TRR1) are also required for protection against a reductive stress induced by exposure to dithiothreitol (DTT). This sensitivity to reducing conditions is not a general property of mutants affected in redox control, as mutants lacking components of the glutathione/glutaredoxin system are unaffected. Furthermore, TRX2 gene expression is induced in response to DTT treatment, indicating that thioredoxins form part of the cellular response to a reductive challenge. Our data indicate that the sensitivity of thioredoxin mutants to reducing stress appears to be a consequence of elevated glutathione levels, which is present predominantly in the reduced form (GSH). The elevated GSH levels also result in a constitutively high unfolded protein response (UPR), indicative of an accumulation of unfolded proteins in the endoplasmic reticulum (ER). However, there does not appear to be a general defect in ER function in thioredoxin mutants, as oxidative protein folding of the model protein carboxypeptidase Y occurs with similar kinetics to the wild-type strain, and trx1 trx2 mutants are unaffected in sensitivity to the glycosylation inhibitor tunicamycin. Furthermore, trr1 mutants are resistant to tunicamycin, consistent with their high UPR. The high UPR seen in trr1 mutants can be abrogated by the GSH-specific reagent 1-chloro-2,4-dinitrobenzene. In summary, thioredoxins are required to maintain redox homeostasis in response to both oxidative and reductive stress conditions.