In an attempt to understand the way automatic cells in the sinus node (SN) control the cardiac rhythm, we studied extracellular electrograms recorded from the SN region in conscious dogs. A SN electrode, containing 48 silver terminals arranged 1.5 mm apart, was implanted over the node, and an indifferent electrode was implanted on the superior vena cava. Through terminals of the SN electrode paired with the vena caval electrode, "unipolar" electrograms were recorded at 100 /iV/cm and with a time constant of 0.1 second. Low amplitude and low frequency deflections (dV/dt ⩽ 20 mV/ sec) which resulted from electrical activity of the node could be differentiated from the more rapid deflections due to atrial electrical activity. Electrical activity due to the inherent automaticity of what appeared to be groups of automatic cells was recognized as a slow negative-going diastolic slope followed by a slow negative-going, or negative and then positive-going, SN potential. Impulse propa-gation toward the SN electrode terminal in groups of automatic cells appeared as a slow positive-going deflection interrupting the diastolic slope. Adjacent groups of automatic cells located near the sites of earliest atrial activation discharged asynchronously before the earliest atrial activity; this suggests that multiple groups of automatic cells might initiate atrial activation. In addition to changes in rate and in location of the pacemaking groups of automatic cells, significant beat-to-beat variation in the sinoatrial interval contributed to the changes in atrial rate in "sinus arrhythmia." These studies provide a better understanding of SN function in conscious animals.Circ Res 47: 775-791, 1980