The influence of exercise on free-radical chemistry is not well understood. It is yet to be confirmed whether an adequate biochemical defence system exists in the human body to provide protection from oxy-centred radicals generated by exercise. Fifty trained elite cyclists undertaking exhaustive endurance training were compared with a control group of 50 sedentary workers. Serum malondialdehyde (MDA), uric acid, superoxide dismutase (SOD), catalase, vitamin E, vitamin C and susceptibility to oxidative stress were assessed. Exhaustive exercise resulted in significantly (p<0.05) higher concentrations of serum MDA, vitamin E and vitamin C, significantly (p<0.001) higher SOD activity, but less significantly (p<0.01) higher concentrations of uric acid and significantly (p<0.05) lower catalase activity in elite cyclists than in the controls. Alterations in the activities of erythrocyte scavenger enzymes (SOD) and higher level of non-enzymatic defences in trained subjects may not be sufficient to counteract the increase in reactive oxygen species produced by endurance training.