The article considers the changes of gas composition, acid-base blood balance, lipid peroxidation processes, and activity of the antioxidant defense system in rat myocardium in the course of adaptation to physical activity (swimming). It has been found out that during the first five days physical activity is accompanied by hypoxia, acidotic blood changes, and increase of lipid peroxidation processes in myocardium. Adaptation to swimming activity (15-30 days) leads to hypoxic and acidotic blood changes, and increases antioxidant defense system in myocardium.