The aim of the present study was to evaluate the changes in glutathione redox ratio (GSSG·GSH^-1) in red blood cells (RBCs) and whole blood in well-trained men following a ski marathon. 16 male subjects (27.0 ± 4.7 yrs, 1.81 ± 0.06 m, 77.6 ± 9.6 kg, VO₂max 66.2 ± 5.7 ml·kg^-1·min^-1) were examined before the competition (pre- COMP), after the competition (post-COMP) and during an 18-hour recovery period (RECOV). There was a slight decrease in reduced glutathione (GSH) in blood and in RBCs in post-COMP. During RECOV, the GSH level in blood was reduced, the GSH level in RBCs was significantly elevated (a statistically significant difference as compared to the pre-COMP level). The post-COMP GSSG·GSH^-1 in full blood did not increase significantly, but its increase was statistically significant during the 18-hour recovery period. During the post-COMP and RECOV, the GSSG·GSH^-1 in RBCs slightly decreased in comparison with the pre-COMP. Vitamin C concentration in serum increased in post-COMP (49% vs. pre- COMP) and decreased to the baseline level during RECOV. In conclusion, our data show that acute exercise slightly increases the GSSG·GSH^-1 in whole blood, while GSSG·GSH^-1 in RBCs significantly decreases. Thus, exercise-related changes in the non-enzymatic components of the glutathione system (GSSG and GSH) in whole blood and RBCs are not identical.

• The glutathione system is a principal cellular non-enzymic antioxidant system in the organism. Long-term or high-intensity exercise may lead to a decreased level of reduced glutathione (GSH), and thereby increase the glutathione redox ratio (GSSG·GSH).

• Limited data are available about the glutathione redox (GSSG·GSH) status measured simultaneously in red blood cells (RBCs) and blood concerning acute high-intensity exercise.

• Acute high-intensity exercise slightly increases the GSSG·GSH in whole blood, while GSSG·GSH significantly decreases in RBCs.

• Our descriptive data show that exercise-induced changes in the non-enzymatic glutathione system seem to be more effective in RBCs and may prevent the damages resulting from reactive oxygen species during exercise.