The interrelationship of spike potentials of "local" intraatrial and esophageal electrograms and notched P wave of an intensified ECG recorded from the body surface was studied. It was found that the process of excitation in the region of the sinus node is reflected in the slow deviation of the P wave of intensified ECG, preceding the appearance of the P wave of an ordinary ECG recorded from the body surface. The A1 spike of a "local" electrogram reflecting the penetration of excitation into the right atrium is manifested in the form of the first notch of the P wave on an intensified ECG. The A2 spike reflecting the arrival of the excitation to the atrioventricular node is manifested in the form of a notch--the first peak of the P wave. The A1(2)) spike reflecting the arrival of the excitation to the left atrium along Bachmann's bundle is manifested as the second peak of the P wave (A1(2)). The A3 spike of the "local" electrogram reflecting the arrival of the excitation from the left atrium to the orifice of the coronary sinus is manifested in the form of the second notch of the P wave (A3).