One or several brief episodes of myocardial ischemia (ischemic preconditioning; IP) rapidly induces tolerance to a later ischemic challenge. This endogenous cardioprotective effect is characterized by a slower onset of cell death. A key feature and probable proximate mechanism of IP is reduced ischemic energy demand which is evident by slower use of ATP and slower accumulation of ischemic catabolites. Several mechanisms for IP and the associated metabolic slowing have been studied: The mitochondrial ATPase is a major cause of ATP hydrolysis in ischemic myocardium but slower ATP depletion in preconditioned myocardium is not due to persistent inhibition of this ATPase. Brief episodes of ischemia in dogs induce stunning as well as IP. Stunning, however, is neither necessary nor sufficient to establish the protective effects of IP. Release of norepinephrine from adrenergic cardiac nerves causes beta adrenergic receptor-mediated stimulation of adenylate cyclase, which stimulates energy-dependent processes. However, IP in dogs that were depleted of catecholamines by pretreatment with reserpine was less effective than IP in control hearts. Thus, an antiadrenergic mechanism does not fully account for the preconditioned state. Another proposed mechanism involves earlier or more complete opening of ATP-sensitive potassium (KATP+) channels. Which of these (or other) pathways mediate the energy sparing effects of ischemic preconditioning remains unknown.