Increased aerobic metabolism during exercise is a potential source of oxidative stress. In muscle, mitochondria are one important source of reactive intermediates that include superoxide,hydrogen peroxide, and possibly hydroxyl radical . The recent discovery that mitochondria may generate nitric oxide also has implications for oxidant production and mitochondrial function. In this review, we critically examine the concept that production of reactive intermediates increases during exercise. Because the health benefits of regular exercise are well-documented, we also examine adaptations to exercise that may decrease oxidative stress. These include increased antioxidant defenses, reduced basal production of oxidants, and reduction of radical leak during oxidative phosphorylation.