We studied the effect of fasting and swimming stress on a number of non-enzymatic and enzymatic antioxidant factors in various mouse tissues in order to see if their action was synergic. We examined levels of reduced (GSH), oxidized (GSSG) and total glutathione, total SH groups (TSH), sum of GSH and protein sulphydryl groups of cytosolic fractions, and the activities of superoxide dismutase (SOD), catalase, glutathione peroxidase, glutathione reductase in adductor muscle, heart and liver. We also studied blood levels of GSH and glutathione bound to protein by mixed disulphides (GSSP). The case series consisted of four groups of animals (n = 10 for each group), namely no swimming and no fast, no swimming and fast, swimming and no fast, and swimming and fast. Fasting (18 h) resulted in a significant GSH depletion in all of the organs studied (-39% in the liver, -30% in the adductor muscle, -21% in the heart); GSSG increased significantly in the heart (+19%). Swimming to exhaustion, which lasted 3.95 (0.18) min [mean (SD), n = 10] with no significant difference between fast and no fast, resulted in a significant GSH depletion, to a percentage lower than that observed after fasting, in the adductor muscle and heart (-12% and -11%, respectively). In the blood of swimming mice, significant increases in GSH (+10%) and GSSG (+21%) levels were observed, whereas GSSP decreased (-15%). Enzyme activities after swimming were modified in only a few cases, and in a complex way. The findings of GSH depletion and a decrease in SOD activity in the adductor muscle seems to confirm the sensitivity of this organ to an overproduction of reactive oxygen species. At the same time, the GSSP decrease observed in blood was a new and unexpected finding, one that indicates a very prompt adaptation of red cells to increased oxidant states.