Reflex heart block was studied in 20 dogs anesthetized with sodium pentobarbital and in 5 trained unanesthetized dogs. Three different vagal reflexes were produced: the Marey response during hypertension caused by administering methoxamine, a cardiogenic hypertensive chemoreflex activated by injection of serotonin into the left atrium and the Hering-Breuer reflex observed during normal respiration of unanesthetized dogs. In every dog during any of the three reflexes heart block was consistently observed after the normal slowing response of the sinus node had been selectively eliminated by the direct perfusion of 10 μg of atropine into the sinus node artery. This was a uniform response despite its being variously produced by a pressor reflex, a chemoreflex or an extracardiac bron-chopulmonary reflex. Transient heart block is therefore to be anticipated during reflexes with vagal efferent components if for any reason the sinus node is incapable of slowing suitably. The possible clinical relevance of these experimental observations is discussed. © 1980.