Quantitative studies of ventricular fusion beats were made in 40 patients with Stokes-Adams disease after implantation of electric pacemakers in the left ventricle. The two fusing complexes came from an electric pacemaker and either a second electric pacemaker or a conducted sinoatrial beat. The limit of fusion, that is the maximal time between two fusing complexes, gives a measure of the conduction time and velocity between the two points of initial ventricular depolarization.Conduction between two electric pacemakers and in retrograde direction from an electric pacemaker to the point of entry of a conducted beat was by way of ordinary myocardium; its velocity was about 1 meter per second. Conduction in antegrade direction, on the other hand, was by way of specialized Purkinje tissue; and its velocity was 4.5 to 9 times faster.Variations in conduction times with bundle-branch block support this interpretation: retrograde conduction time did not change; antegrade conduction time was prolonged in left but was usually unchanged in right bundle-branch block.The morphologic similarities of fusion beats in our patients to the features of anomalous AV excitation add strong evidence in man to the previous analogous experimental observations that the WPW complex is a fusion beat. Furthermore, quantitative comparison of WPW and our fusion beats supports the theory that the anomalous excitation in the WPW phenomenon, like the peripherally located artificial stimulus, enters the ventricle peripherally, by way of an accessory AV pathway.