Purpose Cryoablation is used in the treatment of atrial fibrillation and other cardiac arrhythmias. This study evaluated a novel 10-cm flexible nitrous oxide cryoprobe in an ovine model of atrial ablation. Description Six sheep were anesthetized, underwent a left thoracotomy, and were placed on cardiopulmonary bypass. A left atriotomy was performed, and the cryoprobe was applied endocardially for 120 seconds at less than –40°C to 4 sites on the left atrium. The atrium was closed and the animals were allowed to recover. After 30 days, the animals were euthanized. Transmurality was evaluated in 5-mm sections of each lesion using 2,3,5-triphenyltetrazolium chloride (TTC) and Masson's trichrome staining. Evaluation All animals survived. One hundred four of 106 sections (98%) were transmural by TTC; 103 of 106 (97%) sections were transmural by trichrome staining. There was no late atrial perforation, intraluminal thrombus, or thromboembolism. Conclusions The device reliably produced transmural lesions in a chronic ovine model. Its performance was equivalent to that of other nitrous oxide cryoablation systems.