The catabolism of high-energy phosphates (HEP) during long-term preservation of donor hearts was investigated in mongrel dogs. Hearts were explanted after being randomly assigned to one of the following groups. Group I: normothermic ischaemic arrest (n = 6), group II: hypothermic ischaemic arrest (n = 6), group III: cardioplegic arrest with NIH cardioplegia (n = 6) and group IV: cardioplegic arrest with UW solution (n = 6). After explantation all hearts were stored cold for 24 hours, achieving a myocardial temperature of 0,5 degrees C. HEP content (ATP and creatine phosphate) and catabolites were determined from serial left ventricular biopsies taken before explantation and during cold storage. A significant decrease of HEP content was found in all groups. In group I and II a significant decrease of HEP was found after one hour of cold storage. After 4 and 8 hours cold storage ATP content was significantly higher in group II. Cardioplegic arrest of dog hearts resulted in a significantly delay of HEP depletion. In group III a significant decrease of ATP was only seen after 12 hours and in group IV already after 6 hours of cold storage. Hearts preserved with the newly developed UW solution showed significantly lower ATP values after 10, 12 and 24 hours than NIH preserved hearts. It is concluded that UW solution is less advantageous than NIH solution in terms of HEP preservation.