The significance of electrophysiological investigation of the heart for the diagnosis and treatment of cardiac rhythm disorders is shown. The results of the study of electrophysiological mechanisms and of diagnostic criteria of the more frequent reciprocal tachycardias are analysed: sino-atrial, atrial, atrioventricular nodal, tachycardias in Wolff--Parkinson--White syndrome. Ventricular tachycardia, and also disorder of the rhythm caused by automatic activity of the ectopic focus. Possibilities of the method in the study of the function of the conductive system of the heart and the assessment of the antiarrhythmic drugs are stressed. The problem of the differentiated treatment of cardiac rhythm disorders is discussed. The role of the electrophysiological investigation in the choice of individual medicinal therapy and in the pre-operation study of patients with cardiac arrhythmias is stressed.