Reperfusion after 2 h of coronary artery occlusion has been shown to result in depressed coronary blood flow to the reperfused region and elevated regional myocardial extraction. This suggests that reperfused myocardium, even after 4 h of reperfusion, possesses limited flow and O2 consumption reserves. We studied the capacity of reperfused myocardium to elevate regional blood flow and regional O2 consumption in response to sustained increased O2 demand, produced by atrial pacing. Two groups of anesthetized open-chest dogs were subjected to 2 h of left anterior descending coronary artery occlusion followed by 4 h of reperfusion. One group was subjected to atrial pacing (40% increase in heart rate) during the entire 4-h reperfusion period. Regional O2 saturation was measured by microspectrophotometry in samples of reperfused and nonoccluded subepicardium and subendocardium, which were taken at the end of the reperfusion period. In the paced group, regional blood flow (radiolabeled microspheres) to reperfused myocardium was significantly higher than to corresponding regions of unpaced hearts (110 +/- 22 vs. 40 +/- 9 ml.min-1 x 100 g-1 in the subendocardium). In the control group, O2 extraction of reperfused subendocardium was significantly higher than that measured in the corresponding nonoccluded region (11.0 +/- 0.9 vs. 8.0 +/- 0.6 ml O2/100 ml). Pacing did not elevate O2 extraction of reperfused myocardium (8.7 +/- 0.6 vs. 8.3 +/- 0.7 ml O2/100 ml). Myocardial O2 consumption was significantly elevated in all regions of the paced heart. It is concluded that reperfused myocardium possesses significant unutilized O2 supply and consumption reserves.