The aim of this study is to assess whether oxygen supply is sufficient to induce normoxic conditions in isolated rat hearts. Hearts are perfused with a Krebs medium supplemented with 11 mM glucose, 0.6 mM lactate, 0.06 mM pyruvate, non delipidated albumin (0.1 mM fatty acids), and either 1.78 mM or 0.76 mM free calcium, at 10 ml.min-1. Graded hypoxia is induced by a stepwise decrease of partial pressure of oxygen (PO2) from 660 to 52 mmHg. Contractile performance, oxygen uptake and lactate plus pyruvate balance are assessed. With high calcium, left ventricular developed pressure, dP/dt max and oxygen uptake increase linearly with PO2 up to 660 mmHg; heart rate increases with PO2 up to 250 mmHg and then tends to stabilize. With low calcium, all parameters reach a plateau over 400 mmHg. Lactate plus pyruvate production suggests a stimulation of glycolysis with high calcium, even at 660 mmHg; conversely, there is no lactate plus pyruvate production with low calcium over 250 mmHg. In conclusion, our results demonstrate that, under a high level of calcium at a constant flow of 10 ml.min-1, cardiac function is always limited by O2 supply, except for heart rate. This raises the question as to the definition of a normoxic state. The better preservation of heart rate during hypoxia, compared to other dynamic parameters, could be explained by a contribution of glycolytic ATP.