Myocardial hypoxia induced by intermittent cross-clamping of the aorto-coronary bypass during closed-heart surgery provokes progressive derangement of the myocardial metabolism. The principal changes observed after 18 min of hypoxia (without protection) were a marked swelling of mitochondria, distortion of cristae, reduction in glycogen content, interstitial oedema, focal destruction of I-band material and coagulation necrosis of myofilaments. Another characteristic change is the decoupling of oxidative phosphorylation. These morphological and biochemical changes were successfully prevented by the administration of Solcoseryl (Actihaemyl), a dialysis concentrate from deproteinised calf blood. The less pronounced structural and functional mitochondrial changes in protected hearts suggest that the myocardium is able to tolerate a more severe hypoxia under Solcoseryl protection.