The purpose of the study was to investigate the relationship between the initial status and the trends of noninvasive electrophysiological predictors of sudden cardiac death (SCD): heart rate variability (HRV), heart rate turbulence (HRT), deceleration capacity (DC) and microvolt T-wave alternans (mTWA) in patients receiving ethacizine with drug effect, and side effects in patients with cardiac arrhythmias without structural heart disease. Material and methods. The study included 67 patients (mean age 51.5±17.1 years) with frequent premature ventricular (PVCs) (40 patients) and premature atrial contractions (PACs) (6 patients), paroxysmal atrial fibrillation (AF) (19 patients), atrial tachycardia (1 patient), a combination of PVCs and AF (1 patient). All patients were examined to exclude the structural structural heart disease. Patients received ethacizine 75—150 mg/day. At baseline and during ethacizine therapy, cardiac arrhythmias, HRV, HRT, DC, and mTWA were evaluated using Holter monitoring. Results. Marked effect was observed in 68% of patients with PVCs and in 50% of patients with supraventricular arrhythmias. Side effects were reported in 31% of patients, drug withdrawal was required in 15% of patients. In 3 patients, onset of Brugada syndrome was reported during ethacizine therapy. During ethacizine therapy, HRV values (SDNN and pNN50) significantly decreased due to anticholinergic action of the drug. HRT changes were insignificant. Mean DC values significantly increased, probably due to the elimination of long-term bigeminal episodes. MTWA changes in the overall group were insignificant during ethacizine therapy. However, baseline mTWA values were significantly higher in patients with effective therapy. The maximum mTWA values (second lead) higher than 29 mV and mTWA values measured at 05.00 AM (first lead) higher than 10 mV predicted ethacizine efficiency with sensitivity of 79.4% and 67.6%, and specificity of 75% and 83.3%, respectively. In patients with effective therapy mTWA significantly decreased during ethacizine treatment (p=0.016). In the group without side effects, baseline SDNN values were significantly higher than in patients with adverse events. SDNN less than 118 ms predicted the adverse effects of the ethacizine, with a sensitivity of 75.6% and specificity of 50%. Conclusions. The non-invasive electrophysiological predictors can be used to predict the efficacy and safety of the ethacizine therapy. SDNN less than 118 ms predicted the adverse effects of the ethacizine, with a sensitivity of 75.6% and specificity of 50%. HRT decrease during ethacizine therapy is explained by the anticholinergic effect and confirms the usefulness of the drug in patients with baseline vagotonia. MTWA determination before therapy allows to predict its efficiency with a sensitivity of 68—79% and a specificity of 75—83% (depending on the mTWA values used). In patients with the marked anti-arrhythmic effect of the drug, mTWA significantly decreased.