Activation of cardiac muscle is mediated by the His-Purkinje system, a discrete pathway containing fast-conducting cells (Purkinje fibers) which coordinate the spread of excitation from the atrioventricular node (AV node) to ventricular myocardium . Although pathologies of this specialized conduction system are common in humans, especially among the elderly , their molecular bases have not been defined. Gap junctions are present at appositions between Purkinje fibers and could provide a mechanism for propagating impulses between these cells . Studies of the expression of connexins - the family of proteins from which gap junctions are formed - reveal that connexin40 (Cx40) is prominent in the conduction system . In order to study the role of gap junction communication in cardiac conduction, we generated mice that lack Cx40. Using electrocardiographic analysis, we show that Cx40 null mice have cardiac conduction abnormalities characteristic of first-degree atrioventricular block with associated bundle branch block. Thus, gap junctions are essential for the rapid conduction of impulses in the His-Purkinje system.