During extracorporeal circulation (ECC) controlled hypothermia is a common method of myocardial protection due to a reduction of the myocardial oxygen consumption. Although the beneficial aspects of hypothermia on the myocardial metabolism have been widely demonstrated the effect of hypothermia on the myocardial oxygen tension (PmyO 2) is unclear. For this reason the PmyO 2 of German Landrace pigs (male, three months of age) during ECC was analysed under mild hypothermia (32 °C, n=6 pigs) and under normothermia (n=10 pigs, control group) within a time period of 23 min (1400 sec). Flexible invasive Clark type microcatheters were used to measure the PmyO 2 in the beating heart. During normothermal ECC a continuous PmyO 2 increase from 36.5±15.8 mmHg to 52.6±27.2 mmHg (+44.1%) after 1400 sec was measured (p=0.02). In contrast, mild hypothermia caused a continuous PmyO 2 decrease from initially 46.9±17.5 mmHg to 36.7±20.8 mmHg (-21.8%, p<0.013) in the test period. Electrocardiography revealed no signs of ischemia or arrhythmia during normo-and hypothermic ECC. It seems obvious that mild hypothermia results in a reduction of the oxygen transfer to the myocardial cells and that this effect outweighs the beneficial effects of hypothermia in the myocardium which are related to reduced oxygen consumption. However, in mild hypothermia oxygen supply to the myocardium remained sufficient for normal myocardial function.