We have studied the effect of the xanthine-oxidase inhibitor allopurinol on the performance of hypoxically stored rat hearts to evaluate the drugs' potential value in heart transplantation. Hearts were stored for 2 hours at 15 degrees C after potassium chloride cardioplegia. Hemodynamics were measured before hypoxia and after reperfusion. Hearts were treated with allopurinol in the cardioplegia, at the time of reperfusion, or with superoxide dismutase (SOD) and catalase (CAT) and compared with control hearts that received no treatment. Hemodynamic performance after hypoxia was significantly improved in the SOD/CAT group compared with the control group. Allopurinol-treated hearts were also significantly improved, especially when the drug was given in the reperfusion phase. Our results demonstrate the importance of free radicals during reperfusion of hypoxically stored hearts. Allopurinol and SOD/CAT may be of value in reducing such damage after heart transplantation.