Various energy sources have been used to """"""""ablate"""""""" (coagulate) myocardium responsible for initiating or sustaining ventricular tachycardia. We propose to further refine a catheter system that initially detects the location responsible for the arrhythmia, and subsequently treats the area with an interstitial dose of diffused laser diode energy. Lasers have not been widely used to treat arrhythmias due to the high cost and complexity of the laser source, and lack of a laser catheter capable of creating a reproducible tissue response without directly radiating blood, forming coagulum or perforating the heart. The proposed technologies overcome these deficiencies. Experiments are designed to enhance delivery and facilitate catheter positioning, add safety features, further understand wavelength and dosimetry effects, improve mapping capability, and enhance deployment ergonomics. The catheter designs will be refined, fabricated, and tested to allow electrophysiologic monitoring, with subsequent ablation, when the source of tachycardia is located. This coagulation will be induced interstially by inserting a diffusing fiber intramyocardially. The diffusing tips spread the light in a cylindrical pattern which has advantages over bare end fibers currently being used. Recent innovations have enabled us to make reproducible, clinically relevant thermal lesions, percutaneously in a canine model, providing proof of principle of the applicability of the proposed technique.