In a two year period the effect of carotid sinus pressure on auriculoventricular conduction in a 70 year old man with angina pectoris increased markedly. At first, right carotid sinus pressure gave only P-R interval prolongation. Two years later, pressure on either side produced dropped beats. The maximum effect on the sinoauricular rate remained the same. No other change could be detected in the patient's heart during this period. The site of the cause of the increased vagal effect was either in the intrinsic cardiac nerves to the atrioventricular node or in the node itself.