Background - Theoretical models suggest that an electrical stimulus causes regions of depolarization and hyperpolarization on either side of a myocardial discontinuity. This study determined experimentally whether an artificial discontinuity gives rise to an activation front in response to an electrical stimulus, consistent with the creation of such polarized regions. Methods and Results - After a thoracotomy in six dogs, a 504-unipolar- electrode plaque was sutured to the fight ventricular epicardium to map activations. From a line electrode parallel to one side of the plaque, 10 S1 stimuli were delivered, followed by S2 and S3 stimuli (S1S1, S1S2, S2S3 interval=300 ms). S1 and S3 stimuli were 25 mA; 5-ms S2 stimuli of both polarities were initially 25 mA and increased in 25 mA increments. The plaque was removed, and a transmural incision was made through the ventricular wall in the middle of the mapped region and sutured closed. The plaque was replaced and the stimulation protocol repeated. Before the incision, S2 stimuli directly activated tissue only near the stimulation site. An activation front arose at the border of the directly activated region and propagated across the plaque. As the S2 stimulus strength was increased, the size of the directly activated region increased. After the incision, sufficiently large S2 stimuli caused direct activation of tissue adjacent to the transmural incision as well as at the stimulation site. Activation fronts that arose adjacent to the transmural incision either propagated proximally toward the stimulation site and collided with the activation front originating from the stimulation wire or propagated distally away from the incision. Minimum S2 stimulus strengths activating areas adjacent to the incision were only 45±14% (cathode) and 39±18% (anode) of the strengths required to directly activate the same area before the incision was formed (P<.05). Conclusions - Myocardial discontinuities can give rise to activation fronts after a stimulus, suggesting the presence of polarized regions adjacent to the discontinuity.