The purpose of this study was to determine the effect of ischemic preconditioning on the changes in interstitial fluid (ISF) purine metabolites and lactate during prolonged regional myocardial ischemia. The study consisted of two groups of anesthetized dogs: a control group (n=13) that was exposed to 60 minutes of regional myocardial ischemia and a preconditioned group (n=10). In the preconditioned group, regional myocardial ischemia was induced for two 5-minute episodes, each followed by 10 minutes of reperfusion. These preconditioning episodes were followed by 60 minutes of sustained regional ischemia. Cardiac ISF was sampled by microdialysis probes implanted in the left ventricular myocardium; dialysate levels served as indices of ISF concentrations. In the preconditioned group, dialysate concentrations of adenosine, inosine, hypoxanthine, total purines, and lactate increased during each of the 5-minute ischemia episodes, with further increases occurring during the first 5 minutes of reperfusion. However, the increases in dialysate purine metabolites during the first period of ischemia/reperfusion were greater than those that occurred during the second ischemia/reperfusion. In addition, preconditioning reduced the rate of ISF purine metabolite and lactate accumulation during the prolonged ischemia when compared with nonpreconditioned animals. These data suggest that preconditioning reduces the energy imbalance that occurs during subsequent myocardial ischemia and thereby diminishes the rate of net adenine nucleotide degradation and cellular production and release of purine metabolites and lactate. In addition, the increase in ISF adenosine seen during the preconditioning episode is consistent with the notion that preconditioning-induced cardioprotection is mediated in part by the action of adenosine at extracellular A1adenosine receptors. (Circulation. 1994;89:2283-2289.)