The effects of Ca2+ ions were studied on the canine heart in situ, submitted to vagal influence or not. In addition to heart rate, conduction time was determined separately in atrial muscle, atrioventricular node and His-Purkinje system by means of His bundle potential recording and the effective refractory period (ERP) measured in atrioventricular node, atrial and ventricular muscle according to the extrastimulus method. In the presence of acetylcholine (ACh) released by vagal endings, an increase in plasma calcium concentration from 2.40 to 4.12 mmol/l, acutely induced by the infusion of calcium chloride, elicits the following alterations: slowing down of sinus rate, lengthening of conduction time in atrioventricular node without change of this time in atrial muscle and His-Purkinje system, prolongation of atrioventricular node ERP, but notable decrease of atrial muscle ERP and slight decrease of ventricular muscle ERP. These effects are similar to those of ACh:Ca2+ ions probably enhance the responsiveness to ACh.