Effect of diazepam on the AV nodal conductivity was studied using a direct perfusion technique of the canine AV node artery in situ. As compared to acetylcholine which induced a rapid onset of third degree AV block with prompt recovery, diazepam injected into the AV node artery in doses from 300 micrograms to 3 mg elicited first to third degree AV blocks of longer duration following gradual prolongation of PR interval. The response was not affected by bilateral vagotomy and sympathectomy. Pretreatment of the AV node with hexamethonium or atropine also failed to modify the diazepam-induced AV blocks. Thus, diazepam seemed to suppress conductivity of the AV node by a direct action. The AV nodal tachycardia induced by infusion of norepinephrine was readily antagonized by a single injection of diazepam, but norepinephrine failed to reverse the AV block induced by diazepam infusion.