In order to assess parameters of heart rate variability (HRV) and prevalence of autoantibodies against the beta(1)-adrenoreceptors in patients with cardiac arrhythmias we studied 42 patients with arrhythmias and 20 healthy control subjects. Thirty one patients with idiopathic arrhythmias were included in group I: with paroxysmal atrial fibrillation or flutter (n=13), paroxysmal atrial tachycardia (n=2) and paroxysmal ventricular tachycardia (n=16). Group II was formed of 11 patients with paroxysmal ventricular tachycardia and dilated cardiomyopathy or chronic myocarditis. ab1-AR were determined in blood serum by direct immunoassay. Synthetic fragment containing 26 amino acids of ab1-AR second loop was used as antigen. Groups I (54.8%) and II (63.6%) showed similar prevalence of ab1-AR, which was significantly higher than in control subjects (10%) (p<0.005). HRV parameters in I group were lower in ab1-AR-positive compared with ab1-AR-negative patients. At the same time HRV parameters in ab1-AR-positive patients were significantly different from those in controls (p<0.05). In group II HRV parameters of ab1-AR-positive and ab1-AR-negative patients were significantly lower than in control subjects (p<0.05). We suppose, that ab1-AR could participate in dysfunction of chronotropic heart regulation and contribute to development of arrhythmias in patients with structurally normal hearts.