Low cardiac output after heart surgery occurs more frequently in infants than in adults. This study was designed to determine whether this finding could be explained by a greater susceptibility of the immature heart to ischemia. An isolated working heart model was used to compare myocardial recovery in sets of hearts from six immature (2 weeks, 500 g) and six mature (20 weeks, 2 kg) rabbits after 10, 20, and 30 min of ischemia at 37 degrees C. Mean aortic pressure (MAP), aortic flow (AF), heart rate (HR), left atrial pressure (LAP), and ATP and glycogen levels were measured before and after ischemia. Hemodynamic results are expressed as the percentage recovery of preischemic values. ATP and glycogen are reported as micrograms per gram dry weight. After each period of ischemia, the immature hearts had superior recovery of AF (95 +/- 7.0, 72 +/- 8.8, 70 +/- 7.5 vs 58 +/- 7.1, 34 +/- 15.5, 13 +/- 9.1, P less than 0.05). After 10 min of ischemia, recovery of MAP was not different (97 +/- 1.5 vs 100 +/- 3.5), but after 20 and 30 min of ischemia, the immature hearts had better recovery of MAP (108 +/- 10.8, 98 +/- 5.4 vs 64 +/- 10.8, 48 +/- 6.0, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)