In the heart in situ of vagotomized dogs, atrioventricular conduction was studied by the His bundle potential recording, sinus rate continuously registered and the effective refractory period (ERP) of the atrial muscle measured by the extrastimulus method. The modifications induced by the acute lowering of plasma potassium concentration from 3.5 to 2.0 mmol/l obtained by hemodialysis appeared to be similar to those due to parasympathetic stimulation and the effects of hypokalemia and acetylcholine (ACh) on the atrioventricular (A-V) and sinoatrial nodes as well as on the atrial contractile tissue gave rise to potentiation. Intraaortically injected near coronary ostia in a dose lower than liminal dose, ACh enhanced to a large extent all the phenomena elicited by hypokalemia, since the variations respectively observed under the influence of hypokalemia alone and the combination of hypokalemia and ACh were as follows: lengthening of conduction time in the A-V node by 100 and 180%, reduction of sinus rate by 10 and 20%, shortening of the atrial ERP by 20 and 40%.