The hypothesis of an oxygen limited thermal tolerance due to restrictions in cardiovascular performance at extreme temperatures was tested in Atlantic cod, Gadus morhua (North Sea). Heart rate, changes in arterial and venous blood flow and venous oxygen tensions were determined during an acute temperature change to define pejus (= getting worse) temperatures which border the thermal optimum range. An exponential increase in heart rate occurred between 2 and 16°C (Q₁₀ = 2.38 ± 0.35). Thermal sensitivity was reduced beyond 16°C when cardiac arrhythmia became visible. Flow weighted MRI measurements of temperature dependent blood flow revealed no exponential but a hyperbolic increase of blood flow with a moderate linear increase at temperatures above 4°C. Therefore, temperature dependent heart rate increments are not mirrored by similar increments in blood flow. Venous P_O2 which reflects the quality of oxygen supply to the heart of cod (no coronary circulation present), followed an inverse U-shaped curve with highest P_VO2 levels at 5.0 ± 0.2°C. Thermal limitation of circulatory performance in cod set in below 2°C and beyond 7°C, respectively, characterised by decreased venous P_O2. Further warming led to a sharp drop in P_VO2 beyond 16.1 ± 1.2°C °C in accordance with the onset of cardiac arrhythmia and likely, the critical temperature. In conclusion, progressive cooling or warming brings cod from a temperature range of optimum cardiac performance into a pejus range, when aerobic scope falls before critical temperatures are reached. These patterns might cause a shift in the geographical distribution of cod with global warming.