After finding the landmarks of the sino-atrial node in the dog by means of serial sections, the authors have recorded the electrical activity in the region of the sinus by means of bipolar and monopolar electrodes placed in contact with it. a)A preatrial wave has been recorded, and its origin is discussed. b) Mechanical destruction of the sino-atrial node is followed by the disappearance of this wave and the appearance of a slow junctional rhythm with a retrograde atriogram. c) When lignocain is applied directly to the sino-atrial node, the pre-atrial potential decreases and then disappears and the post-stimulation pause is prolonged. d) Clamping of the peri-sinus region gives rise to an incomplete block, followed by complete block, of the pre-atrial potential and the P wave. The post-stimulation pause remains normal. The disorders of rhythm which have followed these producedures may be regarded as constituting experimental models for sinus lesions and for sino-atrial block.