Strenuous exercise is characterized by increased oxygen consumption and the disturbance between intracellular pro-oxidant and antioxidant homeostasis. Although there are several studies related to an increase in antioxidant enzyme activity in adults doing exercise, the effect of regular exercise on antioxidant enzymes and lipid peroxidation levels has not been examined in children. In our study, the effects of a four week regular swimming exercise on antioxidant enzymes (superoxide dismutase and glutathione peroxidase) activities in erythrocytes and plasma thiobarbituric acid reactive substances (TBARS) levels, an indicator of lipid peroxidation, were investigated in previously untrained healthy children. We found that superoxide dismutase (SOD) activity was increased significantly following a four week swimming course (from 581.1 +/- 146.2 to 791.1 +/- 221.9 U/gHb, P < 0.01). Conversly, plasma TBARS levels were decreased from 1.1 +/- 0.4 to 0.9 +/- 0.3 nmol/ml (P < 0.05). Glutathione peroxidase (GPx) activity appeared to increase following swimming course, albeit not statistically significant (from 45.5 +/- 16.5 to 50.3 +/- 14.8 U/gHb). According to these findings, regular swimming exercise has beneficial effects on antioxidant defence in healthy children.