To detect maturational differences with ischemia/reperfusion injury on cardiac metabolism and function, isolated nonworking rabbit hearts were subjected to 30 min of 37 degrees C ischemic arrest and reperfusion. Pre- and postischemic high energy phosphate contents (ATP, ADP, AMP), conjugated diene (products of free radical mediated lipid peroxidation) production, and peak isovolumic developed pressure (PDP) were measured in newborn (3-5 days, n = 8), 2- to 3-week-old (n = 8), and adult (2-4 months, n = 8) rabbits. ATP content decreased significantly during ischemia in all three age groups but recovered significantly toward preischemic levels in the newborn and 2-week-old groups but not in adult animals. AMP was much better preserved in the two younger groups with significantly higher levels at end-ischemia. Conjugated dienes were present in newborn and adult heart in small amounts at preischemia and rose slightly by end-ischemia. Newborn hearts accumulated large amounts of dienes by 10 min of reperfusion, which were significantly greater than those adult hearts. PDP returned to 85 and 91% of control in newborn and 2-week-old hearts, respectively, and to only 66% of control in adult hearts (P less than 0.05). These data suggest that the postischemic immature heart recovers energy stores and ventricular function faster than the adult heart which can be attributed to preservation of the total adenine nucleotide pool during ischemia. This improved recovery occurs despite a greater amount of free radical-mediated lipid peroxidation with reperfusion in newborn hearts.