NONPERMANENT bundle branch block is a frequent electrocardiographic finding. Lewis1recorded it first, and the literature concerning it has been reviewed by Sabathié,2Rasario,3Yater4and Comeau and associates.5Numerous synonyms have been applied to this condition, each synonym indicating somewhat different circumstances of occurrence. The terms transient,6paroxysmal,7incomplete,8inconstant9and functional10have been applied to the brief episodes of impaired bundle branch conduction appearing in the course of tachycardia or auricular arrhythmias,11coronary occlusion or insufficiency,12congestive heart failure,13thyrotoxicosis,14pulmonary embolism,15infectious diseases,16uremia,17congenital heart disease,18transient hypertension19and quinidine or digitalis administration.20Infrequently there is no detectable cardiac disease.21When no cardiac disease exists, the block has been designated "functional bundle branch block," a term also applied to the Wolff-Parkinson-White syndrome.22For clarity, the discussion which follows excludes specifically the consideration of the last-named entity. Intrinsic heart disease is present in