The aim of this study was to follow up whether the modification of pro-antioxidant status by 8-day oral application of N-acetylcysteine (NAC) in healthy men affects the haematological response, whether there is a direct relationship between antioxidant defences and erythropoietin (EPO) secretion and whether NAC intake enhances exercise performance. Fifteen healthy men were randomly assigned to one of two groups: control or NAC (1,200 mg d(-1) for 8 days prior to and 600 mg on the day of exercise trial). To measure the ergogenic effectiveness of NAC, subjects performed incremental cycle exercise until exhaustion. NAC administration significantly influenced the resting and post-exercise level of glutathione (+31%) as well as the resting activity of glutathione enzymes (glutathione reductase, -22%; glutathione peroxidase, -18%). The oxidative damage markers, i.e., protein carbonylation and lipid peroxidation products (thiobarbituric acid reactive substance) were reduced by NAC by more than 30%. NAC noticeably affected the plasma level of EPO (+26%), haemoglobin (+9%), haematocrit (+9%) and erythrocytes (-6%) at rest and after exercise. The mean corpuscular volume and the mean corpuscular haemoglobin increased by more than 12%. Plasma total thiols increased by 17% and directly correlated with EPO level (r = 0.528, P < 0.05). NAC treatment, contrary to expectations, did not significantly affect exercise performance. Our study has shown that 8-day NAC intake at a daily dose of 1,200 mg favours a pro-antioxidant status and affects haematological indices but does not enhance exercise performance.