A guiding catheter for delivery of intravascular devices to a patient's coronary sinus. The guiding catheter has a relatively stiff proximal section and a relatively flexible distal section, the latter being about 1 to about 6 inches in length and being configured to subselect a branch cardiac vein leading to the coronary sinus. The guiding catheter is particularly suitable for delivering an intravascular device for sensing electrical activity into a cardiac vein to detect such activity from within the blood vessel.