The potential adverse effect of 1) electrical nonuniformity induced by right ventricular pacing and 2) mechanical nonuniformity observed in a model of stunned myocardium induced by 1 hour of severe coronary artery stenosis on left ventricular contraction and relaxation was studied in conscious dogs instrumented for long-term measurements of global and regional left ventricular function. Altering the sequence of electrical activation resulted in a significant change in contraction and relaxation predominantly through asynchrony rather than through changes in inotropy or loading conditions. In the presence of abnormal depolarization, sympathetic stimulation induced by treadmill exercise does not restore synchrony but, on the contrary, results in a significant prolongation of relaxation constant T1. During the early phase of reperfusion after 1 hour of severe coronary artery stenosis, left ventricular relaxation constant T1 is prolonged, whereas at the same time, temporal and regional asynchrony between the normal zone and the stunned zone are significantly augmented. The similarity between the time course of recovery of both global left ventricular relaxation and asynchrony suggests that asynchronous contraction, rather than regional relaxation abnormalities of the stunned myocardium, is the major cause for prolonged relaxation constant T1 during recovery from myocardial ischemia.