Preliminary experiments with cardiopulmonary bypass showed that satisfactory preservation of the heart could be obtained for periods of up to 4 hours ischaemic using single dose cold cardioplegia and rapid induction of local profound cardiac hypothermia. Thereafter morbidity from prolonged cardiopulmonary bypass became a limiting factor. The present experiments were designed to test the efficacy of cardioplegia and rapid cooling for prolonged (16 hour) periods of myocardial ischaemia using orthotopic heart transplantation in pigs as the experimental model. The donor heart is excised after infusing cold cardioplegia into the aortic root during venous inflow occlusion and then rapidly cooled and maintained at 2 to 4°C in cardioplegic solution. Sixteen hours later it is transplanted orthotopically into the recipient animal. Early graft function has been good or excellent in 7 of 8 experiments thus far, as judged by these animals being weaned satisfactorily from bypass with minimal inotropic support. Four pigs subsequently lived between 6 hours and 83 days; the latter animals being sacrificed electively after immunosuppression with Cyclosporin A. This simple and effective method for achieving extended periods of myocardial preservation has important implications with regard to clinical heart donor procurement.