Vectorcardiographic and electrocardiographic data in 4 patients with the Wolff-Parkinson-White syndrome are presented. These studies support the concept that the mechanism responsible for the pre-excitation pattern is a functioning accessory neuromuscular bridge that by-passes the atrioventricular node. Early delivery of the impulse from the sino-auricular node to the lower chambers initiates premature ventricular activation with consequent shortening of the PR interval and lengthening of the QRS interval. Retrograde conduction through the accessory tract may be observed, and is a possible explanation of the atrial arrhythmias that occur in this syndrome.