We examined the activation patterns in the left ventricular epicardial surface of dog hearts using multielectrode grid during the beats initiating ventricular tachycardia 5 to 120 min after coronary artery occlusion. The grid was placed on the ischemic border zone (the area was confirmed by ST-isopotential mapping) to record simultaneously 36 unipolar epicardial potentials. Under the control conditions almost radial spread of activation from the central stimulating electrode of the grid was observed. Activation during acute ischemia showed crowded isochrone lines in the ischemic area, especially during the premature beats with shorter coupling intervals. In 8 out of 15 episodes, where ventricular tachycardia was induced by a single premature stimulus, the conduction delay during premature beats culminated in the local conduction block accompanied by circus movement of activation. Subsequent initial beats of ventricular tachycardia also showed similar activation patterns indicating reentry of excitation. In the remaining 7 episodes, however, we could not obtain such circus movement of activation under the grid. We also measured the effective refractory period at 5 sites under the grid. The results showed greater dispersion of the effective refractory period in the ischemic border zone throughout 5 to 120 min after coronary occlusion in the cases where ventricular tachycardia developed. These findings suggest that reentry of excitation, which is most probably caused by inhomogeneity of refractoriness in the ischemic border zone, may play an important role in the genesis of ventricular tachyarrhythmias during acute ischemia.