Some embodiments of an electrical stimulation system employ wireless electrode assemblies to provide pacing therapy, defibrillation therapy, or other stimulation therapy. In certain embodiments, the wireless electrode assemblies may include a guide wire channel so that each electrode assembly can be advanced over a guide wire instrument through the endocardium. For example, a distal tip portion of a guide wire instrument can penetrate through the endocardium and into the myocardial wall of a heart chamber, and the electrode assembly may then be advanced over the guide wire and into the heart chamber wall. In such circumstances, the guide wire instrument (and other portions of the delivery system) can be retracted from the heart chamber wall, thereby leaving the electrode assembly embedded in the heart tissue.