Four specimens of univentricular hearts of the right ventricular type, three with two atrioventricular valves and one with a common valve, have been examined histologically to identify the atrioventricular conduction system. Three of the specimens were initially thought to have only one chamber within the ventricular mass, a chamber which had a right ventricular trabecular pattern. On detailed examination and with removal of blocks of tissue for histologic study, a second, rudimentary chamber was found in the posterior ventricular wall. This chamber had a trabecular zone of the left ventricular type and was connected with the main chamber only via a small defect in the septum which separated the two chambers. In all four cases the connecting atrioventricular node was dound in normal relationship to atrial markers and the penetrating bundle descended onto the posterior wall of the main chamber. In three cases it ran along the crest of the septum between the main chamber and rudimentary chamber, in relation to the septal defect. In two cases left bundle branch tissue was identified running into the rudimentary chamber. This pattern of conducting system is different from that previously identified in other varieties of univentricular heart and again underlines the importance of intraoperative mapping of the conduction system during surgery of the univentricular heart.