Reports on enhanced nucleotide regeneration by purines during reperfusion are conflicting. We have, therefore, evaluated the effects of inosine or adenine, administered after ischemia, on adenine nucleotide levels and function in isolated rat hearts. The hearts were perfused with a Tyrode solution, containing 10 mM D-glucose, with or without 5 mM pyruvate. After 15 minutes without flow, the hearts were reperfused for 45 minutes with 20 microM purine and 0.5 mM D-ribose. Adenine nucleotide levels tended to recover better in the purine-treated groups. The purines decreased the ATP/ADP ratio by 10-15% (p less than 0.05) if pyruvate was absent. The IMP level in the inosine/glucose group exceeded that in all other groups by a factor of two (p less than 0.001). Inosine increased the adenosine concentration in the effluent sixfold (p less than 0.005). The hypoxanthine concentration rose up to four times following adenine treatment (p less than 0.05). The administration of purine, with or without pyruvate, did not affect mechanical recovery, heart rate or coronary flow. We conclude that inosine and adenine failed to improve cardiac function and hardly affected nucleotide levels in the reperfused heart.