Introduction: The success of radiofrequency energy catheter ablation for the treatment of patients with paroxysmal or chronic atrial fibrillation has been complicated by postablation development of pulmonary vein (PV) stenosis. The acute effect of heat on PV structure has not been well characterized. The aim of this study was to characterize the effects of hyperthermic exposure on PVs and to determine the critical temperature at which PV stenosis would be observed. Methods and Results: The main PVs of acutely explanted canine hearts were identified, isolated, and dissected from the left atria. Measurements of the diameter of 3‐mm PV rings were obtained at baseline and after hyperthermic exposure at 60°C, 65°C, 70°C, and 80°C. Mechanical behavior of PVs was studied by examining the stress‐strain relationship of each PV specimen after heat exposure at different temperature settings. Twenty‐eight PVs samples from 15 dogs (1–3 PVs/dog) were obtained and had a mean circumference of 8.7 ± 2.3 mm at baseline. After heating to 60°C, 65°C, 70°C, and 80°C, the mean PV circumference was 9.2 ± 2.3 mm (P = NS vs baseline), 6.8 ± 2.3 mm (P < 0.0001), 4.2 ± 1.5 mm (P < 0.0001) , and 4.2 ± 0.8 mm (P < 0.0001) , respectively. The stress‐strain curves of PV specimens were shifted to the left upon exposure to higher temperatures, indicating loss of compliance of PVs due to heat exposure. Histology showed loss of typical collagen matrix above 60°C. Conclusion: Significant acute heat‐induced contraction of PVs at zero‐tension state was observed between 60°C and 65°C. This change was associated with collagen denaturation. (J Cardiovasc Electrophysiol, Vol. 14, pp. 250‐254, March 2003)