Cardiac electrical activity during sinus rhythm and episodes of atrial fibrillation were recorded when 42 patients underwent electrophysiologic study procedures. Frequency and time of occurrence, severity of functional symptoms of episodes of atrial fibrillation were evaluated by 24-hour electrocardiogram recordings. Catheter-based endocardial mapping with the assistance of threedimensional electroanatomic mapping systems were conducted to localize the original triggers of atrial fibrillation, to explore electrical activity of atria and ventricles and to measure conduction time within atrial and ventricular tissues. Programmed atrial and ventricular stimulation protocols were performed to study sinus node recovery time and effective refractory periods of atria and ventricles. Results: 100 percent of patients suffered from episodes of atrial fibrillation reported the symptom of palpitations. Other symptoms were fatigue (72.1 percent), chest pain (69.3 percent) and dyspnea at effort (41.9 percent). Severity of symptoms evaluated by EHRA score were 3.19 +/- 0.45. Basic cycle length of sinus rhythm, atria's effective refractory period were significant longer in the subgroup of over 60 year-old patients when compared to under 60 year-old group. 70 percent of patients were determined the location of prematured atrial complexes triggering episodes of atrial fibrillation originating from the pulmonary vein ostia of those 83.3 percent of prematured atrial complexes firing from left superior pulmonary vein whereas 4.8 percent of ones emerging from right atrium.