The association between exercise intensity and related oxidative stress was investigated in nine men who exercised for 30 min at their aerobic (AeT) and anaerobic (AnaeT) thresholds. We also tested the effect of oral N-acetylcysteine (NAC) on exercise-associated rapid blood glutathione (GSH) oxidation in subjects performing two identical maximal bicycle ergometer exercise (Max) tests. Before the second test (Max with NAC supplementation [Max(NAC)]), the men took 200 x 4 mg/day of NAC tablets for 2 days and an additional 800 mg on the test morning. Blood samples were drawn before, immediately after, and 24 h after the tests. Total and oxidized GSH levels in blood were determined. Plasma thiobarbituric acid-reactive substances and net peroxyl radical scavenging capacity (PSC) were assayed. Exercise-associated damage in leukocyte DNA was estimated by fluorometric analysis of DNA unwinding. A single bout of exercise at Max, AeT, and AnaeT resulted in a significant increase in blood GSH oxidation but did not influence net PSC of plasma. Although an association between a single bout of exercise and leukocyte DNA damage was apparent, this study suggests that the parameter may not serve as a sensitive index to assess the role of exercise intensity in the extent of exercise-associated oxidative stress. Plasma thiobarbituric acid-reactive substances did not change after either Max or Max(NAC) tests. NAC supplementation resulted in an increase in preexercise PSC, indicating a higher net antioxidant capacity of the plasma, but did not affect blood GSH.(ABSTRACT TRUNCATED AT 250 WORDS)