1. The automaticity of the A-V junction was evaluated in 15 awake dogs with experimentally induced A-V junctional rhythm. 2. The spontaneous heart rate of these dogs ranged from 54 to 112 beats a minute, showing about 1.5-2.5 times slower than that of control dogs with sinus rhythm. The duration of asystole after overdrive in these dogs prolonged significantly in accordance with an increase in the frequency of stimulation for overdrive, and its mean +/- SD attained 4.7 +/- 1.1 seconds after overdrive at a rate of 2.5 times the spontaneous heart rate. 3. By administration of atropine (0.04 mg/kg, i.v.) to 8 dogs, the duration of asystole after overdrive, at a rate of 1.5 times the spontaneous heart rate, decreased from 2.6 +/- 0.8 to 1.5 +/- 0.4 seconds. By administration of practolol (0.5 mg/kg, i.v.) to 7 other dogs, the duration of asystole after overdrive increased from 3.0 +/- 1.1 to 6.4 +/- 2.2 seconds. 4. It should be suggested that, (1) about 5 seconds of asystole might physiologically occur before the initiation of the A-V junctional escape beat during the long-standing sinus arrest, and (2) the sympathetic nerve might play a more important role in regulating the automaticity of the A-V junction than the vagus nerve.