Atrial Fibrillation is more common in conditions with elevated atrial pressure and can be induced experimentally with acute increases in atrial pressure. We examined the effect of increased atrial pressure with and without pericardial constraint to better separate the effects of increased pressure and atrial stretch. In Langendorff perfused rabbit hearts with intact pericardium, after ligating the pulmonary and caval veins, intra-atrial pressures were increased in a stepwise manner by adjusting the pulmonary outflow cannula Rapid burst pacing was applied to induce atrial fibrillation at increasing intra-atrial pressures from 0 to 24cm H₂0. The atrial refractory period was recorded at each pressure using a single extra stimulus. The protocol was repeated after the pericardium was removed. When the pericardium was intact, atrial stretch was limited by passive constraint and sustained atrial fibrillation could not be induced despite atrial pressures in excess of 20cm H₂0. In contrast, when the pericardium was removed, atrial fibrillation could be reliably induced when atrial pressure exceeded 15cm H₂0. This suggests that the electrophysiological effects of acute atrial volume loading rely on atrial stretch rather than increased atrial pressure alone.