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The hypothesis of an oxygen-limited thermal tolerance was tested in the Antarctic teleost Pachycara brachycephalum. With the use of flow-through respirometry, in vivo³¹P-NMR spectroscopy, and MRI, we studied energy metabolism, intracellular pH (pH_i), blood flow, and oxygenation between 0 and 13°C under normoxia (Po ₂: 20.3 to 21.3 kPa) and hyperoxia (Po ₂: 45 kPa). Hyperoxia reduced the metabolic increment and the rise in arterial blood flow observed under normoxia. The normoxic increase of blood flow leveled off beyond 7°C, indicating a cardiovascular capacity limitation. Ventilatory effort displayed an exponential rise in both groups. In the liver, blood oxygenation increased, whereas in white muscle it remained unaltered (normoxia) or declined (hyperoxia). In both groups, the slope of pH_i changes followed the alpha-stat pattern below 6°C, whereas it decreased above. In conclusion, aerobic scope …