Strenuous exercises greatly increase oxygen consumption in the whole body, especially in skeletal muscles. Large part of oxygen consumption by the body is reduced to H O and ATP, but smaller part (2-5%) of these tremendious increase 2 oxygen consumption results in an increased leakage of electrons from the mitochondrial respiratory chain, forming various reactive oxygen species-ROS (O ˙ˉ, H O i OH˙). TheseROS are capable of triggering a chain of damaging biochemical and 2 2 2 physiological reactions such as oxidative stress and lipid peroxidatio. There is a groving number of informaztions pointing on important role of free radicals as mediators in inflamation and damage of skeletal muscles after strenuous exercise. The mechanism of exercise induced oxidative stress are not well understood. The mitochondrial electron transport chain, polymorphneutrophil and xanthine oxidase have been identified as major source of intracellular free radical generation during and after exercise. Also the interaction of metmyoglobin and methaemoglobin with peroxides may be also an importante source of oxidative stress during exercise.