Reactive oxygen species are potentially harmful compounds which can be generated from molecular dioxygen and other sources in living systems. Aerobic organisms have evolved a number of defense mechanisms for mitigating the effects of reactive oxygen species, including the use of antioxidant enzymes which can degrade reactive oxygen species, and this article discusses work in this and other laboratories on changes in the activities of muscle antioxidant enzymes in response to oxidative stress caused by exercise and hypertension. These studies have shown that various muscles respond differently to oxidative stress in terms of changes in antioxidant enzyme activities and that different control mechanisms for the expression of individual antioxidant enzymes may exist.