It has usually been assumed that when the sino-auricular nodes in the hearts of dogs have been completely destroyed the auriculoventricular node, particularly its coronary sinus portion, becomes the pacemaker or the initiatory mechanism. Eyster and Meek1reported in such cases a decreased p-r interval and a decreased cardiac rate. Borman,2however, obtained normal electrocardiograms from dogs in which the sinoauricular nodes had been destroyed by radon. Therefore, it was our primary object in this study to determine the exact site of the initiation of the impulse in a larger series of dogs in which complete destruction of the sino-auricular nodes was similarly produced by radon. METHOD In a series of thirty dogs, the sino-auricular nodes were aseptically exposed. The only modification of the previously described technic2consisted in the use of capillary tubes containing radon which were either inserted subepicardially by means of a right angle applicator, or as