The combined effects of digoxin and hypercalcaemia were studied in the canine heart in situ on the sinoatrial (SA) and atrioventricular (AV) nodes. Measurements were made of heart rate, of conduction time in the AV node by the endocavitory recording of the His bundle potentials, and of the effective refractory period of this node by the extrastimulus method. In the presence of acetylcholine released by vagal endings or infused into the coronary blood, an increase in plasma calcium concentration from 2.50 to 4.60 mmol/l after a 80 micrograms/kg dose of digoxin considerably depressed conduction in the AV node and automatism in the SA node. In the absence of acetylcholine, no bradycardia occurred under the influence of digoxin alone or digoxin and hypercalcaemia, and hypercalcemia enhanced to a lesser extent digoxin-induced depression of conduction in the AV node. These results evidence a potentiation by acetylcholine of the combined effects of digoxin and hypercalcaemia on the SA and AV nodes.