Using surface and transmission electron microscopy in eight isolated sheep hearts it was noted that cold crystalloid cardioplegic solution produced coronary endothelial damage which could be prevented if homologous blood or albumin was added into the preservation fluid. Similar endothelial damage was observed in the endothelium of aorto-coronary saphenous vein bypasses and in the endothelium of the proximal aorta after infusion with cold crystalloid cardioplegic fluid. No endothelial changes were seen after infusion of the grafts with cold blood. There was no correlation between the preservation of the high energy phosphates of the myocardium and the endothelial changes of the coronary arteries.