The numerous surgical and transcatheter ablation methods widely applicable to the adult heart have had limited use and efficacy in the immature myocardium. This study introduces a more precise modification of the transepicardial approach to ablation, applicable to the immature heart, by combining selective mapping and simultaneous ablation techniques. To determine the specificity of this method, we attempted His bundle and sinus node ablations in 14 beagle puppies (aged 2 to 4 months). After a thoracotomy, a custom bipolar needle mapping/ablation probe, attached to a standard electrophysiologic recorder, was applied along the aortoatrial and atrial-superior vena caval junctions to record maximal epicardial His bundle and sinus node impulses, respectively. Complete ablation was achieved by the single injection of 10% formalin specifically into the desired target region of the conduction system after the probe was advanced into the myocardium proper. Ventricular pacing was then instituted, the thoracotomy closed, and the animals allowed to recover. After 4 months' observation, bidirectional complete atrioventricular block persisted. High right atrial electrical activity was absent in the animals that underwent sinus node ablation. Programmed stimulation failed to induce any arrhythmias. Histologic study demonstrated selective His bundle and sinus node destruction with minimal involvement of surrounding tissue. This study demonstrates a reproducible transepicardial approach to precise ablation in the immature heart.