Long-term preservation of dog hearts was performed over 24 h using Bretschneider-HTK cardioplegia and cold storage. Preservation was assessed in terms of conservation of myocardial tissue levels of high-energy phosphates (HEP) and functional outcome after cardiac transplantation. Serial left ventricular biopsies were taken and analysed for ATP, ADP, AMP, adenosine, inosine, hypoxanthine, xanthine and creatine phosphate. Myocardial structure was studied by electron microscopical examination of a similar biopsy specimen. Cardiac performance was measured before and after cardiac transplantation. Several techniques of cardioplegic arrest were studied: single dose cardioplegia, multidose cardioplegia and continuous perfusion with the cardioplegic solution. In all groups, the hearts were stored at 0.5 degree C for 24 h. In the group of single dose Bretschneider-HTK cardioplegia, myocardial ATP content after 24 h of cold storage was only 25% of control. The total sum of nucleotides at that time interval was however 65% of the control value. Reperfusion of these hearts using a support dog (whole blood reperfusion) did not result in any recovery of ATP. Creatinine phosphate however showed an overshoot. Accumulated nucleosides were washed out. The hearts showed electrical activity but were severely arrhythmic. Contractility was poor. In the group of multidose Bretschneider-HTK cardioplegia, HEP preservation was better than after single dose cardioplegia. ATP content was about 50% of control. The total sum of nucleotides was 85% of control. Ultrastructural assessment of the myocytes revealed only slight ischaemic damage to the mitochondria. Reperfusion on cardiopulmonary bypass after cardiac transplantation did not show any restoration of ATP, but a steady catabolism of HEP. The nucleosides adenosine and inosine were not washed out upon reperfusion. After cardiac transplantation, none of these hearts could be weaned from cardiopulmonary bypass due to irreversible low cardiac output. Histological examination demonstrated irreversible myocardial tissue damage. In the group of continuous cold Bretschneider cardioplegia, HEP content was completely preserved throughout the 24 h of perfusion. Ultrastructure of the myocytes was normal. Reperfusion of the transplanted hearts showed a mild breakdown of ATP to 70% of control values accompanied by a slight accumulation of nucleosides. Haemodynamic recovery however was perfect and none of the hearts needed positive inotropic support. Myocytes after reperfusion had a normal subcellular appearance.(ABSTRACT TRUNCATED AT 400 WORDS)