The events associated to myocardial ischemia result from 2 overlapping phenomena due to reduced blood flow (ischemia) and reduced O2 supply (hypoxemia). To distinguish these effects, 2 groups of isolated rat hearts were perfused through the aorta (Langendorff's method) with Krebs-Henseleit buffer, and were exposed for 20 min to hypoxemia or ischemia, matched in terms of the O2 supply (10% of baseline), with continuous monitoring of cardiac contractility, O2 uptake and lactate production. The developed pressure and the O2 uptake were similar in hypoxemic and ischemic hearts; heart rate, end-diastolic pressure and lactate production rate were higher in hypoxemia than in ischemia; the recovery from hypoxemia was less than that from ischemia, despite the same O2 supplies; treatment with superoxide dismutase and catalase, scavengers of the O2 derived free radicals, during hypoxemia, allowed hypoxemic hearts to recover as ischemic hearts. Therefore, the main determinant of the reperfusion injury is to be attributed to the low O2 supply rather than to the low coronary flow; part of the injury is due to free radicals; a substantial portion is mediated by the energy demand during the stress which was higher in hypoxemia than in ischemia.