JENKINS, R. R. and A. GOLDFARB. Introduction: oxidant stress, aging, and exercise. Med. Sci. Sports Exerc. Vol. 25, No. 2, pp. 210–212, 1993. In recent years research in the basis and applied sciences has broadened our understanding of oxygen chemistry and its influences on biological systems. Although living organisms are endowed with a broad array of biochemical defense mechanisms for protection against potentially harmful radical chemistry, oxidative stress may overwhelm those defenses. It is now clear that exercise may initiate oxidative stress. It is the aim of this symposium to review and broaden our understanding of the problem of free radical chemistry as it applies to the exercise milieu.