Isolated canine hearts were preserved for 12 h at 5 degrees C followed by normothermic reperfusion for 2 h. Dogs were divided into two groups: group 1 (n = 7) received a nondepolarizing preservation solution in multidose, and group 2 (n = 6) received single-flushed University of Wisconsin (UW) solution, both administered in multidose fashion. At the end of reperfusion, the myocardial adenosine triphosphate concentration and left ventricular systolic and diastolic function were preserved better in groupl than in group 2. Myocardial mitochondrial ultrastructural integrity was identical in the two groups. These results suggested that in a 12-h heart preservation, nondepolarizing solution administered in multidose fashion protects the myocardium from the deleterious effects of hypothermia and cardioplegia better than UW solution.