The purpose of this study was to compare the degree of ischemic and hypoxic injury in normal versus hypertrophied rat hearts to investigate basic mechanisms responsible for irreversible myocardial ischemic injury. Hearts from rats with bands placed on the aortic arch at 23 days of age (BAND) and sham-operated rats (SHAM, 8 weeks postoperative) were isolated, perfused with Krebs buffer, and had a left ventricular balloon to measure developed pressure. Hearts were made globally ischemic until they developed peak ischemic contracture and were reperfused for 30 minutes. Additional hearts were perfussed for 15 minutes with glucose-free hypoxic buffer followed by 20 minutes of oxygenated perfusion. There was an 87% increase in heart weight of BAND compared with SHAM (p < 0.01). During ischemia, lactate levels increased faster in BAND compared with SHAM, ischemic contracture occurred earlier in BAND than in SHAM despite no difference in ATP levels, and postischemic recovery of left ventricular pressure was less in BAND (26.8 ± 5.6% of control left ventricular pressure, mean ± SEM) compared with SHAM (40 ± 4.6%, p < 0.05). During hypoxic perfusion, lactate release was greater in BAND than in SHAM (48.8 ± 1.2 versus 26.6 ± 0.97 μmol/g, p < 0.01), and with reoxygenation, lactate dehydrogenase release was less in BAND than in SHAM (13.2 ± 0.7 versus 19.5 ± 0.2 IU/g, p < 0.01). After hypoxia and reoxygenation, left ventricular pressure recovery was greater in BAND than in SHAM (93 ± 8.4% versus 66 ± 5.3%, p < 0.01). Thus, this study suggests that hypertrophied hearts have a greater potential for glycolytic metabolism, resulting in an increased rate of by-product accumulation during ischemia, which may be responsible for the increased susceptibility of hypertrophied hearts to ischemic injury.