The sinus node is the normal pacemaker in the mammalian heart. Except in rare circumstances, the resting heart rate is determined by a succession of synchronized impulses generated by some or all of its cells capable of spontaneous depolarization. When the sinus node defaults, the major substitute pacemaker to come to the rescue is located in the atrioventricular (AV) junctional region. Recently, several studies, have demonstrated that there are atrial pacemakers that are not strictly confirmed to either the sinus node or the AV junctional region. Both recent and old experimental evidence indicate that left atrial ectopic pacemakers are rare and that the vast majority of ectopic centers reside in the right atrium. This leads us to consider a few embryologic features of the heart.