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Mongrel dogs were chronically implanted with sonomicrometer crystals and a high fidelity tip micromanometer to measure ventricular wall thickness (WT) and left ventricular pressure (LVP), respectively. Two weeks after surgical operation, the left circumflex coronary artery was occluded with a hydraulic occluder for 3 minutes in conscious condition. During reperfusion, the hemodynamic parameters and systolic wall thickening recovered to the normal level quickly, while an abnormal thickening phase during early relaxation (extra phase) appeared in the dWT/dt-WT loop (X-axis = WT, Y-axis = dWT/dt) with its pattern different from that during control and ischemic conditions. This kind of pattern of the loop could be observed during hypoxia and during rapid overfilling of coronary artery. It is supposed that some substances, which may be produced in the ischemic myocardium, induced an extra dilation of coronary artery during reperfusion and a rapid overperfusion in the early relaxation phase of the cardiac cycle, leading to the abnormal pattern in the loop.