Transmyocardial revascularization (TMR) has been investigated as a means of reperfusing ischemic heart tissue by introducing channels in the myocardium. In percutaneous TMR (P TMR), the channels are made from within the left ventricle by introducing an excimer laser catheter via the femoral artery. There is currently no means of determining the limits of ablation in P TMR so that the epicardium is not perforated. We used an optical coherence tomography (OCT) signal to detect the epicardial surface during excimer laser ablation of the myocardium in porcine hearts. A XeCl excimer laser operating at 308 nm with a pulse duration of 125 - 200 ns and a pulse energy of 21/mJ at 25 Hz was used to ablate channels in porcine hearts ex vivo. The OCT system was connected to an optical fiber guidewire that was introduced coaxially with the ablation fiber. The OCT signal was monitored on a computer during the ablation process. The epicardial surface was detected within 600 micrometer of perforation.