Abstract: Pre-shaped cardiac catheters for mapping and selective ablation of a portion of cardiac circuitry includes a preshaped first curved portion for positioning around the ostium of coronary sinus and a second curved portion for maintaining the first curved portion in its desired position. The method involves introducing a catheter assembly, including a guide-wire and a preshaped catheter, to a location proximal the atrium. As the guide-wire is withdrawn from within the catheter, the catheter assumes its preshaped form at the target location. Alternatively, a catheter assembly, with or without a guide-wire, may be introduced to the target ablation site via a catheter sheath. The catheter includes an array of spaced apart electrodes on at least a portion of the catheter. Each electrode may be activated remotely to a preselected current level for a preselected time interval to ablate a target portion of cardiac circuitry adjacent the electrodes.