The current views of the authors regarding the relationship of the cardiac performance and its myocardial oxygen consumption and the relationship of the latter to coronary blood flow are presented. Recent attempts, including our own, to devise an index of total cardiac effort that will relate to the myocardial oxygen consumption are discussed. It is concluded that the cardiac performance requiring oxygen determines coronary flow while the arteriovenous oxygen difference across the bed remains constant. However, the level of the arteriovenous oxygen difference is altered by local conditions of oxygen and carbon dioxide concentrations, and the presence of catecholamines. Hence, the level of coronary flow for a given amount of cardiac oxygen consumption is altered by these circumstances.