As well as transmitting the impulse from the atria to the ventricles the atrioventricular node has two other important functions namely: synchronisation of atrial and ventricular contractions by a varying delay; and protection of the ventricles from rapid atrial arrhythmias. The relative importance of these two functions appears to differ in large and small mammalian hearts. In small mammals synchronisation of atrial and ventricular contractions is the major function of the atrioventricular node, whereas protection from rapid atrial arrhythmias may be its most important function in large mammals. Atrioventricular conduction time in sinus rhythm is ten times longer in the whale (500 ms) than in the rat (50 ms). A whale heart, however, is about 100 000 times heavier than a rat heart. During atrial fibrillation the ventricular rate in a dog heart is only three times faster than in a horse, whereas a horse heart may be 40 times as heavy as that of a dog. Hence there is a considerable discrepancy between the size of the mammalian heart and the functions of its atrioventricular node. Analysis of several anatomical and functional aspects of atrioventricular conduction systems in mammals of all sizes showed that the importance of the delaying role of the atrioventricular conduction system diminishes as the size of the mammal increases, whereas the protective role of the atrioventricular node probably increases. The function of the human atrioventricular node seems to be intermediate between that of of the small and large mammals.