This study evaluates the effects of epicardial RF ablation on the coronary vessels in a canine model. Nonsurgical epicardial catheter ablation is a minimally invasive procedure that has proven to be efficacious for the treatment of VT. This approach is limited by concern regarding the potential adverse effects of RF ablation on the epicardial coronary arteries. After lateral thoracotomy, a multipolar linear ablation catheter was sewn adjacent to or crossing the LAD in nine mongrel dogs. Five of these dogs also had a standard 4-mm tip ablation catheter sewn adjacent to the LAD. RF pulses were delivered using temperature control. Animals were sacrificed 14 days after ablation for histological analyses. Using the 4-mm tip catheter, 22 consecutive single lesions of 2.67 +/- 0.35 mm depth were studied. The only coronary arterial change seen was replacement of the media with extracellular matrix. Using the linear ablation catheter, 117 pulses were applied to generate 24 linear lesions (3.8 +/- 1 mm depth). Replacement of the coronary arterial media with extracellular matrix proliferation was commonly seen, and severe hyperplasia occurred in one artery. Intravascular thrombosis occurred in six arteries. The internal perimeter of the vessel (0.78 +/- 0.49 mm vs 1.79 +/- 0.83 mm) was the only variable associated with severe arterial damage. The effects of RF ablation delivered adjacent to the LAD were limited to the media but when delivered above the artery, severe intimal hyperplasia and intravascular thrombosis may occur. Susceptibility to damage is inversely proportional to the vessel size.