Objective. To evaluate electrocardiographic (ECG) markers of electrical instability of the myocardium in patients suffering from paroxysmal atrial fibrillation and receiving preventive antiarrhythmic therapy. Material and methods. The following parameters in patients with coronary heart disease, functional class II–III stress angina, and paroxysmal atrial fibrillation were analyzed: heart rate variability, high-resolution ECG, the Q–T interval during Holter ECG monitoring. The results were evaluated in groups depending on the preventive antiarrhythmic drug: allapinin at a daily dose of 75 mg (n=16), allapinin combined with metoprolol at a dose of 75 and 50 mg per day (n=18), allapinin combined with sotahexal at a dose of 75 and 160 mg per day (n=18), as compared with the healthy (n=24) and patients who suffered from ischemic heart disease, but did not have heart rhythm disorders (n=24). The examination was conducted using the expert system «Inkart» KT-07-3/12 P. Results. The most significant changes of heart rate variability in the form of reductions in standard deviation from all the analyzed RR intervals (SDNN) by 36% and 23%, as compared to the healthy, and by 37 and 25%, as compared to the control group, were detected in patients receiving allapinin monotherapy and allapinin combined with sotahexal. Late ventricular and atrial potentials were recorded with higher frequency against the background of the combination of allapinin and sotahexal, while late atrial and ventricular potentials with even higher frequency and their combinations occurred in the allapinin monotherapy group. Conclusion. First, in the case of paroxysmal atrial fibrillation against the background of allapinin monotherapy, a decrease in indicators reflecting the overall heart rate variability as well as the high frequency of late ventricular potentials and their combination with late atrial potentials is detected. Second, when allapinin is combined with sotahexal to prevent atrial fibrillation paroxysms, a decrease in both the overall heart rate variability in terms of SDNN and the high frequency of late atrial potentials are detected. Third, the optimal combination for the prevention of atrial fibrillation paroxysms is that of allapinin with metoprolol, in which late ventricular potentials are detected less frequently and late atrial potentials are not detected.