In 16 traces of spontaneous activity of sinus node pacemaker cells of various species from mouse to man, the charge change during repolarisation was between 3 and 8 times (mostly between 3 and 4 times) that recorded during the pacemaker current ("funny current”, If). This finding is incompatible with the necessity for a steady state of ionic movement in and out of the cell - the ionic balance; there is a deficit of outward positive ion movement sufficient to complete repolarisation. It is postulated that the deficit is solved by generation of electrical energy in the form of electrons by oxidative phosphorylation of mitochondria.