Acetylcholine (ACh) in concentrations above 0.1 μg/ml causes temporary arrest of the heart since it (1) suppresses sinus node automaticity and (2) blocks sino-atrial conduction. These two factors appear simultaneously and it has been difficult to decide if one or if both of them remain in force throughout the period of atrial arrest. By using microelectrodes in the isolated, perfused canine sinus node we recorded transmembrane potentials during normal sinus rhythm, atrial pacing, and atrial arrest. Automaticity (the rate of subthreshold impulse generation) recovered within the first minute after ACh but sinoatrial conduction of the impulses remained suppressed; action potential amplitude was less than 30 mV. Sinoatrial conduction was restored 1 ± 0.5 min later as the sinus node action potentials reached amplitudes exceeding 30 mV. Therefore, the chronotropic and dromotropic effects of ACh appear to be two separate changes, each being demonstrated independently. © 1981.