The phenomenon of supernormality of myocardial excitability and conduction has been recognized for many years but somewhat neglected in recent literature. Supernormal excitability can be demonstrated occasionally in patients with pacemakers and high myocardial thresholds at an interval of 400 to 520 ms after the R-wave. Supernormal excitability is a prerequisite for supernormal conduction which may be observed to follow conducted sinus beats as well as beats of ectopic origin. In the latter case, retrograde concealed conduction must be assumed to be operative. The basis for the detection of supernormality in only occasional, but not all, similarly predisposed patients remains unestablished. The differential diagnosis of supernormality may pose a difficult electrocardiographic problem, the analysis of which should include consideration of the following: simultaneous occurrence of both premature atrial and ventricular beats, AV-dissociation with capture beats, longitudinal dissociation giving rise to varying AV-intervals and bilateral bundle branch block.