Han-Wistar rats were exposed to a 194-200 h swimming protocol which caused a significant increase in the cardiac weight. The levels of various tissue antioxidants were assayed from the myocardium of the right ventricle and from the left ventricle (subendo- and subepimyocardium). This endurance training decreased the activities of catalase in the right ventricle and in the subendo- and subepimyocardium and Cu,Zn-superoxide dismutase in the subendomyocardium as well as the concentration of vitamin E in the right ventricle and in the subendomyocardium. Also, the activity of thioredoxin reductase decreased in each part of myocardium and that of glutathione reductase in the right ventricle and in the subepimyocardium. The activity of glucose-6-phosphate dehydrogenase increased in the right ventricle and in the subepimyocardium. The activity of glutathione peroxidase and the total tissue contents of carnosine and anserine and tissue sulphydryl groups remained unchanged as compared to the control group. The endurance training caused only minor changes in the regional distribution of antioxidants. The major findings were the disappearance of the difference in the activity of catalase between the right and the left ventricle and the increase in the activity of glucose-6-phosphate dehydrogenase as compared to that of the left ventricle. The results show that endurance training by swimming decreases the level of cardiac antioxidants. This decrease may be due to the increased oxygen metabolism and the subsequent increase in the formation of oxygen free radicals, which could deplete the antioxidant pool.