The management of 164 patients with a paroxysm of angina pectoris by a cardiologic emergency aid unit included, besides a clinical examination, the recording of the ECG in 12 commonly accepted and additional leads suggested by Neb, N.F. Gurevich, and Slapac--Partilla. The values of the atrial and the end portion of the ventricular complexes of the ECG were analysed. Three groups of patients were distinguished; group 1 with uncomplicated (stable) angina pectoris; group 2 with subacute and group 3 with acute preinfarction state. It was established that the severity of ischemic changes in the left ventricular myocardium and the pronounced character of changes in values charaterizing definite deficiency of left atrial bio-electric potentials correlate and grow successively from group 1 to group 3. Among the patients hospitalized with a state of preinfarction 53.9% developed myocardial infarction; in 89.9% of patients the localization of the necrosis coincided with the ischemic zones in the preinfarction state.