Recent studies implicate oxidative stress in the mechanism of apoptosis. We have examined the expression of genes, whose products counteract oxidative stress, during glucocorticoid-mediated apoptosis of a murine thymoma-derived cell line. Using Northern blot hybridisation analyses, we observed a progressive decline over a 24 h period in the transcript levels for catalase, manganese superoxide dismutase, copper, zinc-superoxide dismutase, DT-diaphorase and thioredoxin. The changes were first seen within 2-8 h of the addition of the hormone which is well in advance of appreciable apoptosis. Using Western blot hybridisation analyses we found that a dexamethasone-mediated increase in glutathione S-transferase message level was followed closely by an increase in glutathione S-transferase mu class protein and a 20% decrease in reduced glutathione levels. Our findings suggest that the downregulation of cellular oxidant defense enzymes with a consequent increase in oxidant damage could contribute to the molecular mechanism of apoptosis.