Cardiac dysrhythmias are a prominent cause of morbidity and mortality. Pharmacological treatment is ineffective in a large number of patients and is associated with many serious side effects. Thus, direct treatment of cardiac arrhythmias has been used with increasing frequency. Each form of direct treatment, such as surgical ablation, DC catheter ablation, radiofrequency catheter ablation, laser catheter ablation suffer serious drawbacks. Thus, we investigated the utility of transvenous catheter cryoablation of the bundle of His in five miniature swine, 40-60 lbs. in weight. Complete atrioventricular block was produced in each animal during cryothermia and persisted for 1 hour of observation in four out of five swine. In the fifth animal, 2:1 atrioventricular block within the atrioventricular node persisted for 1 hour of observation. Morphological and histologic examination revealed no dysfunction of capillaries and myofibriles in the atrioventricular node and proximal bundle of His. This potential mode of transcatheter therapy deserves further investigation.