The pathophysiological cause of the short PR interval usually associated with glycogenosis of the heart (Pompe disease) has not been explained. In an effort to define this abnormality, intracardiac electrocardiography was performed in an 18-month-old child who had histologically confirmed Pompe disease. The low right atrium to His bundle interval (LRA-H), a reflection of atrioventricular nodal conduction time, measured only 40% of the average LRA-H for this age. Treatment of this patient with a glycolytic enzyme lowered the skeletal muscle glycogen level, decreased ventricular electrical forces, and slightly lengthened the PR interval. It is proposed that the excess glycogen caused both the accelerated conduction through the atrioventricular node and the increased QRS forces by a mechanical or chemical action on the myocardial cells.