The effects of neurotransmitters injected into the posterior and the anterior septal artery on the automaticity of the atrioventricular (AV) junctional area were examined in the excised, blood-perfused canine AV node preparation. An AV rhythm of 57.4 +/- 2.8 beats/min (N = 28) developed after the destruction of the sinoatrial node. l-Norepinephrine injected into the anterior septal artery increased the frequency of AV rhythm, but l-norepinephrine injected into the posterior septal artery caused a pacemaker shift from the anterior septal artery area to the posterior septal artery area. Acetylcholine injected into the posterior septal artery blocked retrograde conduction but did not decrease the frequency of AV rhythm; however, acetylcholine injected into the anterior septal artery decreased AV rhythm but did not affect retrograde conduction. Destruction of the posterior septal artery area failed to change AV rhythm. The surgically separated anterior septal artery preparation had the same rate as did the AV node preparation. The posterior septal artery preparation had a definitely lower rate and responded to smaller doses of l-norepinephrine than did the AV node preparation. The results of the present study indicate that AV rhythm in the AV node originates in the area supplied by the anterior septal artery and that the area supplied by the posterior septal artery, which has extremely low automaticity, is highly responsive to l-norepinephrine, resulting in nodal tachycardia.