spontaneous beat was determined. The intensity of the driving pacemaker was adjusted at will. The distance between the driving and testing stimuli was progressively shortened by adjusting the delay time, so as to map out ventricular excitability. The supernormal phase was studied by scanning the cycle, with the intensity of the test pacemaker being slightly below threshold. A careful search for the Wedensky effect was made in all cases. During idioventricular beating the intensity of one pacemaker was kept slightly below threshold.