Stenothermal polar ectotherms show a restricted range of thermal tolerance. According to earlier work in crustaceans and fish (Frederich and Pörtner 2000; Van Dijk et al. 1999), thermal limitation becomes effective first by a drop in aerobic scope at pejus temperatures Tp and, then, by the onset of anaerobic mitochondrial metabolism, which is expressed in the critical temperature Tc.Thus, a hypothesis was set up stating that oxygen limitation characterizes the first line of thermal intolerance. To test this hypothesis, we investigated the effects of hyperoxia and temperature on energy metabolism, blood flow and tissue oxygenation in the Antarctic eelpout Pachycara brachycephalum using in vivo MR methods in combination with oxygen consumption measurements.Surprisingly, exposure to hyperoxia (PO2: 45kPa) and warmer temperatures resulted in a linear increase in oxygen consumption; the typical exponential increment of oxygen uptake was eliminated. Similarly, blood flow in the Aorta dorsalis increased above 6°C in normoxic but not to the same extent in hyperoxic animals. Liver tissue oxygenation increased steadily in both groups from 0-11°C, above which there was a sudden drop in the normoxic controls whilst hyperoxic animals kept a high level of tissue oxygenation.These findings suggest that improved oxygen availability diminishes the effects of thermal stress by reducing the energy cost of oxygen uptake and distribution. Further restrictive mechanisms become effective once the oxygen limitation of thermal tolerance has been eliminated.(1) Frederich, M., Pörtner, H.O. 2000. Am. J. Physiol. 279, R1531-R1538.(2) Van Dijk et al. 1999. J. Exp. Biol. 202, 3611-3621.