Introduction: Production of isoprostanes (IsoPs) is enhanced after acute, intense, and prolonged exercise, in untrained subjects. This effect is greater in older subjects. The present study aims to delineate the profile of acute-exercise-induced IsoPs levels in young and older endurance-trained subjects. Methods: All included subjects were male, young (n ¼ 6; 29 yrs ± 5.7) or older (n ¼ 6; 63.7 yrs ± 2.3), and competitors. The kinetics of F 2-IsoPs in blood-sera was assessed at rest, for the maximal aerobic exercise power (MAP) corresponding to the cardio-respiratory fitness index and after a 30-min recovery period. Results: No significant time effect on F 2-IsoPs kinetics was identified in young subjects. However, in older athletes, F 2-IsoPs blood-concentrations at the MAP were higher than at rest, whereas these blood-concentrations did not differ between rest and after the 30-min recovery period. Conclusion: Because plasma glutathione (GSH) promotes the formation of some F 2-IsoPs, we suggest that the surprising decrease in F 2-IsoPs levels in older subjects would be caused by decreased GSH under major ROS production in older subjects. We argue that the assessment F 2-IsoPs in plasma as biomarkers of the aging process should be challenged by exercise to improve the assessment of the functional response against reactive oxygen species in older subjects.