The purpose of this study was to compare interstitial fluid (ISF) levels of purine metabolites and lactate in the endocardium and the epicardium during graded regional myocardial ischemia and reperfusion. Anesthetized dogs were subjected to 60 min of regional myocardial ischemia induced by either partial or complete occlusion of the left anterior descending coronary artery (LAD), followed by 60 min of reperfusion. To sample ISF, cardiac microdialysis probes were implanted in the LAD-perfused myocardium; dialysate levels served as indexes of ISF concentrations. During severe ischemia, dialysate adenosine increased transiently in both the endocardium and epicardium, reaching maximal values at approximately 20 min of ischemia. Inosine, hypoxanthine, xanthine, and lactate increased most rapidly during the first 30 min of severe ischemia, after which the rate of increase was diminished. The ISF profiles of these metabolites were qualitatively similar during moderate ischemia, although the ISF levels achieved during ischemia were not as great. With both severe and moderate ischemia, ISF purines and lactate were greater in the endocardium than epicardium, consistent with a greater energy imbalance in the endocardium during ischemia. ISF total purines (the sum of the individual purine metabolites) were relatively stable until myocardial blood flow was reduced below 50 ml.min-1 x 100 g-1, after which ISF total purines increased in proportion to the severity of the blood flow deficit. These data suggest that functional and metabolic adaptations keep the myocardium in energy balance until blood flow is reduced below approximately 50% of control, and they attest to the usefulness of cardiac microdialysis for establishing transmural profiles of ISF metabolites.