Antarctic marine ectotherms look back on several million years of adaptation to constant extreme cold temperatures. Most endemic species are cold stenothermal, with low and not thermally compensated metabolic rates, low scope for routine activity and a narrow thermal tolerance envelope (Peck 2002, Polar Biol. 25, 31-40, for rev.). Their being prey to warm blooded top predators creates a need for temperature compensation of burst swimming in polar fishes. Hence, their scope for exercise activity is well comparable to temperate fish (for rev. see Pörtner 2002, J. Exp. Biol. 205, 2217-2230).We have recently reviewed antioxidant capacity in polar and temperate ectotherms (Abele & Puntarulo 2004, CBPA in press) and found a mismatch between thermally slowed standard and routine metabolism and a well compensated antioxidant system in polar invertebrates and fish. This does not only apply to enzymatic antioxidant capacities (SOD, catalase, glutathione peroxidase), but, more importantly, polar animals display far higher glutathione and α-tocopherol tissues concentrations. As oxygen radical formation is largely a function of metabolic activity, the question arises what creates the elevated need for antioxidant protection in cold blooded polar animals?Mechanisms of accelerated ROS production could be rooted in a loss of oxygen buffering by low or absent oxygen binding pigments, and also in higher lipid unsaturation in the cold.Moreover, the fraction of oxygen converted to ROS in mitochondria from polar bivalves is higher than in temperate bivalve mitochondria. Several features, like slow oxygen diffusion and low myoglobin oxygen buffering in muscle of Antarctic ectotherms at habitat temperatures, indicate higher oxidative stress may accompany fatiguing exercise in polar fauna, if compared to similar temperate speciesIn our presentation we will summarize current knowledge to model the mismatch between thermally suppressed routine metabolism and compensated burst activity, as one potential cause of elevated ROS production and the need for sustained high levels of antioxidant defence in polar ectotherms.