Using aseptic methods, with histological and electrocardiographic control observations on 14 animals, the attempt was made to find out how much of the sino-auricular node can be removed and still have the normal pacemaking function retained by the remainder. The method employed was not satisfactory for the purpose, but it was found that a considerable portion of the node could be removed with continuation of function of the remnant. There were marked individual variations in the amount of nodal tissue essential to the preservation of the normal rhythm. Changes in the P-R interval were noted, and in one dog an A-V nodal rhythm persisted for 8 days post-opera-tively, with return to the pre-operative sino-auricular node rhythm.