Isolated canine hearts were preserved for 24 hours by either simple hypothermic storage in Collins' M solution (group 1, n = 7) or hypothermic perfusion with oxygenated modified Krebs solution (group 2, n = 7), followed in both cases by reperfusion with cross circulation. Myocardial biopsies taken consecutively during preservation and after reperfusion were analyzed for high energy phosphates (adenosine triphosphate [ATP], adenosine diphosphate [ADP], adenosine monophosphate [AMP], and creatine phosphate), their catabolites (inosine), and lactate. In group 1, ATP decreased from 23.7 +/- 2.8 mumol/gm dry weight (mean +/- standard deviation) at 0 hours to 5.6 +/- 2.8 mumol/gm dry weight at the end of preservation. Creatine phosphate decreased and lactate increased significantly during 24 hours of preservation. The consumption rate of ATP appeared to have decreased after 18 hours of preservation. In group 2, ATP, creatine phosphate, and lactate remained at control levels during preservation. Coronary vascular resistance, however, increased significantly. After reperfusion functional measurements of the hearts in both groups did not vary significantly, despite the difference in myocardial ATP content (11.2 +/- 4.9 mumol/gm dry weight in group 1 and 17.9 +/- 3.9 mumol/gm dry weight in group 2; p less than 0.05). Simple hypothermic storage manifested a limitation in terms of energy store compared with hypothermic perfusion, whereas the latter appeared to have a risk of possible myocardial damage caused by the perfusion itself.