Exercise induces extensive generation of reactive oxygen species, which are responsible for tissue damage: enzymes inactivation, lipid peroxidation and single strand breaks in DNA. Defense system against free radicals is consisting of enzymes such as superoxide dismutase, catalase, glutathione peroxidase, and numerous non-enzymatic antioxidants. The study was performed to evaluate the effect of a single bout of submaximal running exercise, on the lymphocyte DNA strand breaks and also to test how supplementation with tempol - a membrane-permeable SOD-mimetic (0.2 mmol/kg/day) influences the eventually evoked damage. Male, Wistar rats were challenged with graded 50 min. running on treadmill at intensity up to 75-85% of predicted VO(2)max. The DNA strand breaks in individual lymphocytes were determined by using a gel electrophoretic technique - "comet" assay. We found substantial lymphocyte DNA damage 60 min. after the exercise. Tempol failed to prevent from oxidative damage in rats challenged with exercise. Moreover tempol by itself induced higher DNA damage than the exercise bout.