Some homeothermic animals can survive adverse conditions by hibernation, i.e., by reducing their body temperature in accordance with ambient temperature, thus reducing metabolism and vital functions. Six years ago, the term hibernation was introduced to describe a particular state of the myocardium. Although the meaning is different from that used in zoology and, thus, is misleading, it is used increasingly to describe a condition induced by moderate reduction in coronary flow. Some evidence in the literature suggests that the myocardium can actively reduce its mechanical function as a consequence of reduced coronary flow in order to prevent ischemia-induced injury. It is conceivable that the hibernating myocardium is the result of such a down-regulation of function. The hibernating myocardium can be characterized by decreased function in the hypoperfused area, which still exhibits active metabolism and remains viable. This is in contrast to "stunned" myocardium, which represents dysfunction during postischemic reperfusion, i.e., with coronary blood flow being close to normal. In analogy to hibernation in its original meaning, down-regulation and hibernating myocardium are considered to represent a protective mechanism, because such myocardium can quickly regain its initial function after restoration of physiologic blood flow. Because hibernating myocardium is salvageable, it has to be distinguished from other dysfunctional tissue that has lost its function due to ischemic damage, so that appropriate clinical interventions can succeed in restoring normal coronary blood flow.