Molecular oxygen is an essential nutrient for higher forms of life. In addition to its normal physiological reactions, oxygen and its partially reduced forms can oxidize a variety of macromolecular and simpler compounds in cells and fluids of the body. Such oxidized and peroxidized compounds have been associated with, and may be causally related to, a variety of chronic diseases. As a protection against excessive oxidation, nature has developed a complex set of interactive antioxidant systems. Selected aspects of antioxidant actions are considered in this symposium.