This application proposes to develop and test a unique laser based catheter system for treatment of ventricular and supraventricular arrhythmias. After successful construction of prototypes and demonstration of effective AV conduction modification in SBIR phase l, the following studies are proposed: 1) optimize catheter development for mapping in the left ventricle 2) evaluate catheter in a canine model of ventricular tachycardia (n=24) 3) compare the effect of laser to radiofrequency in a canine model (n=20) 4) optimize catheter development for AV nodal conduction modification in a canine model (n=12) and 5) start a clinical trial initially for supraventricular and later for ventricular tachycardia. The ALS system has a monitoring feedback system that measures laser penetration depth by ultrasound and tissue temperature. ALS catheters use electrode guidance for a side emitting laser beam which travels in a column of saline solution that clears the path between the catheter and arrhythmia focus. The ALS non-contact laser delivery system provides optimal control and maximizes depth of tissue coagulation. The investigators and consultants submitting this proposal are recognized leaders in the field of lasers, catheter engineering and electrophysiology.