The biology of the myocardium was studied under experimental conditions similar to angina pectoris. In some dogs the myocardium was adapted to ischaemia by progressive coronary occlusion of 1-5 min followed by restoration of circulation during 5 min. In other dogs adaption was followed by 20 to 35 min ischaemia. The animals were sacrificed immediately or after 2-10 days. Transient ischaemia produced less severe alterations then abrupt coronary obstruction. Adaptation followed by 20 and 35 min ischaemia induced foci that undergo cytolysis and scarring of maximum intensity on the 8th day. Activity of enzymes in the mitochondrial suspension, especially of cytochromoxidase, decreases and lysosomal hydrolases increase with focal necroses.