We tested the hypothesis that neonatal myocardium is more sensitive to injury during cardiac surgery than adult myocardium. Groups of neonatal (4 to 5 days old) and adult (3 months old) isolated, blood-perfused rabbit hearts were subjected to periods of ischemic arrest and reperfusion without cardioplegic protection as follows. Protocol I consisted of 30 min of normothermic (37 degrees C) ischemic arrest and reperfusion, protocol II of 60 min of hypothermic (15 degrees C) ischemic arrest and warm (37 degrees C) reperfusion, and protocol III of 120 min of hypothermic ischemic arrest and cool (21 degrees to 25 degrees C) reperfusion. After protocol I, the neonatal hearts recovered 23% greater contractile function than the adult hearts; after protocols II and III the neonatal hearts recovered 51% to 55% more function than the adults. These results suggest that neonatal hearts tolerate the conditions of cardiac surgery better than adult hearts.