Two groups (A and B) of isolated baboon hearts were preserved by continuous hypothermic perfusion storage for 48 hours using perfusates that, according to the manufacturers, differed only in the concentrations of the contaminating trace elements iron, lead, and arsenic. Storage with the perfusate containing the higher concentration of these elements (perfusate B) led to significantly less gain in heart mass, a greater reduction in coronary flow, coronary sinus effluent lactate, and myocardial arteriovenous oxygen difference and a greater increase in coronary sinus effluent lactate dehydrogenase, when compared with perfusate A. Group B hearts totally failed to support the circulation following orthotopic transplantation, whereas group A hearts showed excellent function. Group B hearts had undergone the typical changes of enhanced resting myocardial tension during the storage period (before warm blood reperfusion); we proposed that these changes were brought about by the production of superoxide anions and radicals by the higher relative concentration of iron, or a combination of contaminating trace elements, in perfusate B. To confirm that these perfusates did differ significantly in the concentration of these trace elements, in particular with regard to iron, the superoxide anion activity in both solutions was measured and was found to be significantly higher in perfusate B. The addition of superoxide dismutase to both solutions inhibited superoxide anion activity by more than 80%. © Williams & Wilkins 1987. All Rights Reserved.