True supernormality of excitability and conduction has been demonstrated in normal Purkinje fibers in in vitro studies. In the clinical setting, supernormality of conduction is manifested better than expected. This phenomenon is much more common than previously thought, particularly in the presence of certain clinical conditions. If a careful scanning of the cardiac cycle is performed on all patients with intermittent bundle branch block and second degree or advanced infranodal AV block, accessory pathways and mulfunctioning pacemakers, it is anticipated that a much larger amount of supernormal excitability and conduction will be unmasked.