Studies have implicated active oxygen species (AOS) in the pathogenesis of various lung diseases. Many chemical and physical agents in the environment are potent generators of AOS, including ozone, hyperoxia, mineral dusts, paraquat, etc. These agents produce AOS by different mechanisms, but frequently the lung is the primary target of toxicity, and exposure results in damage to lung tissue to varying degrees. The lung has developed defenses to AOS-mediated damage, which include antioxidant enzymes, the superoxide dismutases [copper-zinc (CuZnSOD) and manganese-containing (MnSOD)], catalase, and glutathione peroxidase (GPX). In this review, antioxidant defenses to environmental stresses in the lung as well as in isolated pulmonary cells following exposure to a number of different oxidants, are summarized. Each oxidant appears to induce a different pattern of antioxidant enzyme response in the lung, although some common trends, i.e., induction of MnSOD following oxidants inducing inflammation or pulmonary fibrosis, in responses to oxidants occur. Responses may vary between the different cell types in the lung as a function of cell-cycle or other factors. Increases in MnSOD mRNA or immunoreactive protein in response to certain oxidants may serve as a biomarker of AOS-mediated damage in the lung.