a wide variety of environmental pollutants are capable of inducing oxidative stress. This has received increasing interest in the field of aquatic toxicology in the last decade. Evidence suggests that pollutants in aquatic environments play an important role in the mechanistic aspects of oxidative damage in aquatic species, leading to adverse effects such as cellular damage associated to carcinogenesis and mutagenesis. The imbalance between the genera - tion and the neutralization of free radicals and other reactive oxygen species by antioxidant mechanisms whitin an organism is called oxidative stress. This review summarizes current knowledge and advances in the understanding of such deleterious processes and the role of antioxidant defenses in fishes.