The relationship of changes in ventricular activation patterns and variations in R-wave amplitude on the surface ECG during the hyperacute phase of myocardial ischemia were studied in 9 open-chest dogs. The sum of R-wave amplitude (ΣRWA) changes in surface ECG leads L2, V5 and Frank orthogonal leads X, Y and Z were correlated with changes in the conduction time along the specialized conduction system and in intramyocardial conduction times, as well as with hemodynamic and echocardiographically determined left ventricular dimensional changes. The hyperacute phase of myocardial ischemia induced by a one-stage occlusion of the left circumflex coronary artery was marked by a progressive increase in left ventricular end-diastolic diameter and left ventricular end-diastolic pressure as well as a progressive decrease in cardiac output. At the same time, ΣRWA and intramyocardial conduction time followed a synchronous biphasic pattern. In the first 30 seconds after coronary artery ligation, intramyocardial conduction time in the ischemic zone accelerated to a peak of 11.3% above control (p < 0.001). This acceleration of conduction was followed closely by a decrease in ΣRWA to 16.8% below control (p < 0.001). A second phase ensued, characterized by a gradual slowing of intramyocardial conduction time in the ischemic zone to 135.1% above control (p < 0.001) and a synchronous increase in ΣRWA to 53.1% above control (p < 0.001). Conduction time along the specialized conduction system did not change significantly. Thus, the asynchrony of ischemic ΣRWA alterations with hemodynamic and left ventricular dimensional changes and the similarity of the biphasic responses of ΣRWA to the changes in intramyocardial conduction time in the ischemic area suggest that ventricular activation patterns rather than hemodynamic and intracardiac dimensional changes may play the major role in determining R-wave amplitude responses to acute myocardial ischemia.