Two species of free-living nematodes, Turbatrix aceti and Caenorhabditis elegans, exhibited a marked sensitivity to 3 atm of 100% O₂. Environmental changes in pH and temperature, which altered nematode respiration, resulted in alterations in the survival of these organisms under high pO₂. Levels of defensive enzymes such as superoxide dismutase, catalase, glutathione peroxidase, and dianisidine peroxidase were measured in the two species. No changes in the level of superoxide dismutase or catalase activity were induced by exposure of the nematodes to high pO₂. Manipulation of these two enzymes was however achieved using the inhibitors 3-amino-1,2,4-triazole and diethyldithiocarbamate. 3-Amino-1,2,4-triazole (20 mm) eliminated ⩾80% of the catalase activity in vivo and diethyldithiocarbamate (5 mm) decreased the level of CuZn superoxide dismutase by ⩾70%. Both of these compounds increased the sensitivity of C. elegans to high pO₂ toxicity. Compounds capable of intracellular redox-cycling with O₂⁻-production, such as plumbagin, increased CN⁻-resistant respiration in the nematodes and imposed an O₂-dependent toxicity. These experiments demonstrate the toxicity of intracellular O₂⁻ and H₂O₂ in nematodes and the importance of superoxide dismutase and catalase in providing a defense against these toxic molecules in vivo.