The relative oxidative insult caused by exercise and smoking on biological systems are well documented, however, their cumulative influence needs to be clarified. In order to examine the collective effects of exercise and smoking on oxidant and antioxidant parameters, young male smokers (n=10) and non-smokers (n=10) made to perform a negative slope (10%) cycling exercise for 30 minutes at individual load equivalent to 60% maximal oxygen consumption (VO 2max). Pre- and post-exercise (post-ex) haematocrit, haemoglobin, white blood cells, plasma malondialdehyde (MDA) levels, protein carbonyl formation and non-HDL oxidation, erythrocyte superoxide dismutase (SOD) and glutathione peroxidase (GPX) activities, serum ceruloplasmin (CER) and urinary cotinine concentrations were evaluated. Pre-ex CER and urinary cotinine concentrations of smokers were significantly higher (p<0.05 and p<0.01, respectively) compared to that of non-smokers and pre-ex CER concentrations were significantly correlated with cotinine levels in all subjects (p<0.05). Significant (p<0.01) increases were observed in non-HDL oxidation following the exercise in both groups and the elevations were more pronounced in smokers. Pre-ex SOD and GPX activities were not different between the two groups, however post-ex enzyme activities were significantly reduced in smokers (p<0.05). MDA and protein carbonyl concentrations were not different between the two groups and there were not any significant changes due to exercise.In conclusion, according to the results of the present study, we suggest that erythrocyte antioxidants SOD and GPX and plasma non-HDL are more prone to the possible oxidant damage of acute physical exercise in chronic smokers. ©Journal of Sports Science and Medicine (2003).