ABSTRACT: Isolated, paced, isovolumetrically beating piglet hearts (n = 37) underwent retrograde aortic perfusion with a crystalloid solution during three periods: 1) baseline (coronary perfusion pressure 60 mm Hg), 2) ischemia (coronary flow 10% of baseline for ± 80 min), and 3) reperfusion (perfusion pressure returned to baseline). In one group of hearts, glycolysis (using 3H20 formation from [3H]glucose) was assessed. During baseline, peak systolic pressure (PSP) was 101.1 ± 5.0 mm Hg, end diastolic pressure (EDP) 4.4 ± 0.5 mm Hg, glycolysis 970.5 ± 65.3 nmol/min/gwet‘. and myocardial glycogen 234.8 ± 12.0 μmol/gdry. During ischemia, PSP decreased to 23.3 ± 2.7 mm Hg, EDP increased to 12.3 ± 0.7 mm Hg, myocardial glycogen decreased to 181.5 ± 30.3 μmol/gdry, and lactate (≈154 μmol/gwet) and glycerol (≈930 nmol/gwen) were released. Myocardial contracture correlated with a decrease in lactate release. Glycolysis decreased to ≈400 nmol/min/gwet and remained stable, accounting for ≈50% of the lactate produced. During reperfusion, PSP recovered to 79.8 ± 3.5 mm Hg, EDP 6.6 ± 1.7 mm Hg, and glycolysis 1103.9 ± 81 nmol/min/gwet. In a second group of hearts, with similar mechanical responses, glucose oxidation (using 14CO2 formation from [14C]glucose) was evaluated. During baseline, glucose oxidation was 165.4 ± 15.9 nmol/min/gwet, and correlated closely (r = 0.957) with mechanical activity. With ischemia, glucose oxidation decreased to ≈17 nmol/min/gwet, yet accounted for ≈42% of the ATP produced. Upon reperfusion, glucose oxidation returned to baseline values, but now correlated poorly (r = 0.574) with mechanical activity. We conclude that for neonatal hearts undergoing severe low-flow ischemia: 1) myocardial contracture is associated with a decline in lactate release, implying impaired lactate production and/or clearance; 2) glycolysis may not fully account for the lactate released, suggesting nonglycolytic sources for energy production; 3) glycerol release is stimulated, indicating hydrolysis of triacylglycerols; 4) glucose oxidation provides an important source of ATP; and 5) glycolysis and glycose oxidation return to baseline values upon reperfusion, despite depressed contractile function, indicating dissociation between mechanical and metabolic recovery.