An intraoperative endocardial array of electrodes which permits simultaneous recordings from multiple sites in the intact ventricle is currently in clinical use and has provided the opportunity for exploring electrical ablation as an alternative to standard surgical ablative techniques requiring a ventriculotomy. A variation of the electrical ablative procedure adapted for this intraoperative purpose was studied in seven dogs to determine its long-term electrophysiologic and structural effects on ventricular myocardium. With the dogs on cardiopulmonary bypass, a mesh-covered latex balloon was introduced into the left ventricle via an atriotomy. Five damped sinusoidal discharges (4 of 200 J per 2 electrodes, one of 100 J to the remaining electrode) were delivered to nine silver bead electrodes sutured at 1 cm intervals onto the mesh and arranged in a 2 x 2 cm2 array. At 5-8 weeks following the ablation, no ventricular arrhythmias could be induced in any of the dogs using standard stimulation techniques. The scar produced by the ablation consisted of dense fibrous tissue, and was well demarcated from surrounding normal myocardium. Cellular electrophysiology confirmed that no action potentials could be recorded within the scarred area while at distances of 1 mm from the scar border, normal responses could be elicited from both muscle cells and Purkinje fibers. The adaptation of the described electrical ablative procedure produces an electrophysiologically inert scar.