Unprotected and preserved myocardial ultrastructures were analyzed in prepump and postreperfused states. The experiments were divided into three groups: group 1 – normal hearts functioning 60 min in a special heart-lung model; group 2 – hearts with 30, 60, 90 and 120 min of normothermic ischemia plus 60 min of recirculation; group 3 -hearts with 30, 60, 90 and 120 min of hypothermic cardioplegia plus 60 min of recirculation. Anoxic hearts could be set in motion, but after longer anoxia the symptoms of stone-heart regularly appeared. Reperfusion induced grave ultrastructural changes both in the energy system (mitochondrial edema, rupture and lysis of cristae) and myofibrils (hypercontraction, elongation, Z line anomalies). Cardioplegic hearts showed a dynamic recovery. However, after reperfusion, characteristic sporadic signs of ischemia could always be demonstra