In order to examine the relationship between underlying heart diseases and lesions of the atrioventricular (AV) conduction system in dogs with third-degree AV block, the hearts of 36 dogs in which the arrhythmia had been diagnosed electrocardiographically were studied post mortem. On histologic analysis, underlying heart diseases were observed in 31 of the 36 dogs: lymphocytic myocarditis (13 dogs), endocardiosis of the AV valves (12 dogs), congenital anomalies (two dogs), cardiac tumors (two dogs), and fatty infiltration of the lowermost portion of the atrial septum (two dogs); the remaining five dogs, however, had no remarkable changes. In all of the 31 dogs, loss and disappearance of conduction fibers were found in the AV node (AVN) and its approaches (AVNap), the penetrating (HisP) and branching (HisB) portions of the His bundle, and/or the left and right bundle branches to various degrees. The main location of the severe lesions was as follows: the whole of the AV conduction system in the case of lymphocytic myocarditis; the HisP and HisB in the case of AV valve endocardiosis; the HisP in the case of congenital cardiac anomalies; the AVNap, AVN, HisP and/or HisB in the case of cardiac tumors; AVNap in the case of fatty infiltration. Thus, it is suggested that the pathological process affecting the AV conduction system has caused third-degree AV block and that some relationship between the affected portion of the AV conduction system and the underlying heart disease seems likely.