Age-related differences in susceptibility to ischemia/reperfusion injury and the response to the iron chelator deferoxamine during reperfusion were studied in isolated nonworking rabbit hearts subjected to 30 or 40 minutes of ischemia at 37 degrees C followed by 30 minutes of reperfusion. In the experimental group, hearts received a bolus of deferoxamine just before the moment of reflow, followed by a continuous infusion during the first 10 minutes of reperfusion. Isovolumic systolic (peak developed pressure) and diastolic (diastolic pressure versus balloon volume relationship) function was assessed with an intracavity balloon and incremental volume changes. In separate groups of hearts, adenine nucleotide content (adenosine triphosphate, diphosphate, and monophosphate) was measured before ischemia, at end-ischemia, and 30 minutes after reperfusion. The cardiac function measurements showed that after 30 minutes of ischemia and 30 minutes of reperfusion, peak developed pressure in newborn hearts recovered to 89% +/- 5% of preischemic levels; this recovery was significantly better than that of adult hearts, which exhibited 67% +/- 6% (p less than 0.01) recovery. Deferoxamine significantly improved cardiac function only in adult hearts (p less than 0.01). However, after 40 minutes of ischemia and 30 minutes of reperfusion, peak developed pressure in newborn hearts was reduced to 61% +/- 3% and was not significantly better than that of adult hearts (54% +/- 5%). Deferoxamine significantly improved systolic function in both newborn and adult hearts (p less than 0.01) exposed to 40 minutes of ischemia. Myocardial adenosine triphosphate content fell markedly by the end of 30 and 40 minutes of ischemia in both groups. After 30 minutes of ischemia, newborn but not adult hearts were able to completely recover adenosine triphosphate content by 30 minutes of reperfusion. This advantage was lost after 40 minutes of ischemia. Deferoxamine had no effect on recovery of adenosine triphosphate content in any group. We conclude that (1) newborn hearts recover postischemic function and metabolism faster than adult hearts after shorter periods of ischemia; (2) this advantage is lost as the ischemic period is prolonged; (3) deferoxamine improved postischemic cardiac function after longer ischemic periods, in both age groups, but failed to improve the recovery of myocardial adenosine triphosphate content.