We have previously shown that aspirin induces apoptosis in manganese superoxide dismutase (MnSOD)-deficient Saccharomyces cerevisiae cells cultivated in ethanol medium, and that it exhibits a significant antioxidant effect until the onset of overt apoptosis. We here report that glucose-6-phosphate dehydrogenase activity in these cells is not inhibited by aspirin. However, the reducing power, as measured by the NADPH/NADP(+) concentration ratio, is significantly lower than in wild-type cells. With aspirin, the levels of NADPH, NADP(+) and catalase in MnSOD-deficient cells decrease significantly after 72 h of cultivation, without significant decrease of the NADPH/NADP(+) ratio. This ratio is higher when the cells are grown in glycerol or acetate medium. This seems to prevent loss in viability and induction of apoptosis on treatment with aspirin. Additionally, the glutathione (GSH) level is maintained, but the level of oxidized glutathione (GSSG) increases, leading to a significant decrease in the GSH/GSSG ratio in aspirin-treated cells. This decrease in the GSH/GSSG ratio is much less in cells grown in glycerol medium, while there is an increase in the GSH/GSSG ratio of cells grown in acetate medium. Consequently, the decreased reducing power may be linked to apoptotic induction by aspirin. This occurs independently of the level of reactive oxygen species which, as shown in our previous studies, do not play a primary role in the apoptosis of cells exposed to aspirin. The protective effect of MnSOD appears to be related to the cellular reducing power.