This report documents five cases of supernormal (SN) conductivity in the sense of unexpected normalization of the previously existing bundle-branch block (BBB) and three cases of SN excitability in which impulses delivered to the ventricles propagated to the myocardium only during a well-defined interval in the cardiac cycles corresponding to the U waves of the preceding beat. In none of the patients could SN be abolished by atropine, suggesting absence of an important vagal role. In the five patients with SN conductivity there were three examples of right BBB in which spontaneous, as well as electrically induced premature atrial beats, were followed by normal QRS complexes only if they reached the right bundle during its SN phase. In the other two patients with bilateral BBB, strategically placed sinus P waves and electrically induced atrial impulses were conducted to the ventricles with normal P-His and His-Q intervals. Analysis of the electrocardiograms and His bundle electrograms indicated that SN conductivity resided in the bundle branches, and not in the A-V node. In one of two patients with bilateral BBB and SN conductivity in the right bundle branch, the SN-conducted beats occurred after both left and right ventricular escape beats, but were coupled to the preceding escape beats by longer intervals after left ventricular escape beats than after right ventricular escape beats. The difference was accounted for by the later activation of the right bundle branch when the preceding escape beat originated from the contralateral left ventricle. These observations favor an electrophysiologic explanation for the phenomenon of supernormality which is most probably related to the increased voltage of the transmembrane action potential in the period commonly known as the period of negative afterpotentials. This study provides no support for the hemodynamic or vagal theory of supernormality.