Six horses consisting of 3 cases with sinoatrial (S-A) block and other 3 without any arrhythmias, were studied for cardiopathology. Histopathologically focal lesions of perinodal myocardial ischemia and/or fibrosis in the S-A node area, were found out in each of the horses exhibiting this arrhythmia. In the 3 horses with sinus rhythm, the postmortem coronary arteriography revealed that the S-A node artery arose from both the left and right coronary arteries, and that the inter-arterial coronary anastomoses were located in a part of the S-A node area with the anterolateral region of the right atrium. The angiographic observation led to the suggestion that occurrence of the ischemic and/or fibrotic lesions of the S-A node area in the animals with S-A block seemed to correspond to localization of the inter-arterial coronary anastomoses. Multifocal lesions in the cardiac nerves of both the sympathetic and parasympathetic (vagal) systems, were also pointed out in the cases with arrhythmia. It seemed probable, therefore, that the pathological changes of the S-A node area, their localization and the cardiac nervous disturbances in both the systems were associated with a genesis of S-A block in the horse.