Background:Unlike "conventional" microsecond pulsed electrical fields that primarily target the cell membranes, nanosecond pulses are thought to primarily electroporate intracellular organelles. We conducted a comprehensive preclinical assessment of catheter-based endocardial nanosecond pulsed field ablation (nsPFA) in swine. Methods: A novel endocardial nsPFA system was evaluated in a total of 25 swine. Using either a low-dose (5-second duration) or high-dose (15-second duration) strategy, thoracic veins and discrete atrial and ventricular sites were ablated. Swine were survived for <1 (n=1), ~2 (n=7), ~7 (n=6), 14 (n=2), or ~28 (n=9) days and venous isolation assessed before sacrifice. Safety assessments included evaluation of esophageal effects, phrenic nerve function, and changes in venous caliber. All tissues were subject to careful gross pathological and histopathological examination. Results: All (100%) veins (13 low-dose, 34 high-dose) were acutely isolated, and all reassessed veins (6 low-dose, 15 high-dose) were durably isolated. All examined vein lesions (10 low-dose, 22 high-dose) were transmural. Vein diameters (n=15) were not significantly changed. Of the animals assessed for phrenic palsy (n=9), 3 (33%) demonstrated only transient palsy. There were no differences between dosing strategies. Thirteen mitral isthmus lesions were analyzed and all 13 (100%) were transmural (depth 6.4±0.4mm). Ventricular lesions were 14.7±4.5mm wide and 7.1±1.3mm deep, with high-dose lesions deeper than low-dose (7.9±1.2mm vs 6.2±0.8mm, p=0.007). The esophagus revealed non-transmural adventitial surface lesions in 5 of 5 (100%) animals sacrificed early (2 days) post-ablation. In the 10 animals sacrificed later (14-28 days), all animals demonstrated significant esophageal healing - 8 with complete resolution, and 2 with only trace fibrosis. Conclusions: A novel, endocardial nanosecond PFA system provides acute and durable venous isolation and linear lesions. Transient phrenic injury and non-transmural esophageal lesions can occur with worst case assessments suggesting limits to PFA tissue selectivity and the need for dedicated assessments during clinical studies.