Electrode catheter ablation (ECA) of atrial muscle may be a useful technique for the treatment of drug-refractory supraventricular tachycardias originating in the right atrial free wall (RAFW). We performed this study in order to determine: (1) the safety of electrical shocks applied to the RAFW and (2) the early and late anatomic effects of ECA. Twelve beagle puppies, ranging in age from 2.0 to 7.5 months and weighting 2.3 to 8.0 kg, underwent electrical ablation of the RAFW, using energy doses of 100 to 400 J. At the highest energy doses tested (400 J), one puppy died of refractory ventricular fibrillation and one of low cardiac output. Atrial perforation and cardiac tamponade occurred in two puppies, each of which received one shock of 200 J. The area of myocardial damage following ECA shocks of 150 J was greater than for shocks of 100 J, 94 +/- 14 vs 56 +/- 11 mm2, respectively (p less than 0.02). Acutely, ECA produced transmural hemorrhagic necrosis. Eleven weeks after electrical ablation, atrial fibrosis was apparent at the site of ablation. In conclusion, ECA may be used to fulgurate atrial tissue in the RAFW. We recommend 150 J as a safe upper limit in small subjects, although higher energy doses may not produce cardiac perforation or adverse hemodynamic effects in larger subjects.