The role of acetylcholine (ACh) in the sinus tachycardia which follows vagal stimulation (‘postvagal tachycardia’) was investigated in anesthetized dogs. The stimulation of either vagus was followed by postvagal tachycardia. The bradycardia induced by local application of ACh to the sinus node was followed by sinus tachycardia while local application of methacholine (a predominantly muscarinic agonist) induced only bradycardia. Atropine potentiated the postvagal tachycardia in low doses and blocked it in high doses. The conclusion drawn is that postvagal tachycardia is caused by the release of ACh by either vagus acting on a nicotinic receptor.