Increase in heart rate may be detrimental to atrioventricular conduction. The effects of such an increase were investigated in the paced intact dog heart, by measuring the conduction time in the atrioventricular node (obtained from the His bundle potential recording) and the effective refractory period in this node as determined by the extrastimulus method. Investigations on atrioventricular nodal conduction were performed with and without vagal influence. After central restoration of vagal tone by dextromoramide in anesthetized animals, both parameters appeared to lengthen as the heart rate rose. This lengthening became considerable at the highest rates. After blockade of cholinergic receptors by atropine, no significant change occurred when the heart rate was raised. The prolongation observed under vagal tone was enhanced by the rise in the plasma calcium concentration to 3.10 mmol X l-1 and reduced by a calcium influx inhibitor, verapamil, in 0.2 mg X kg-1 dose. These effects are related to the calcium intracellular concentration, since the rise in this concentration beyond an optimum has been demonstrated to inhibit the calcium and potassium channel.