The purpose of the study was to evaluate the atrial electrical instability in relation to to structural-functional status of myocardium, heart rate variability (HRV), electrophysiological heart peculiarities and some neurohumoral markers in patients with paroxysmal atrial fibrillation (PAF) and or without ischemic heart disease (IHD). 343 patients with PAF of nonvalvular genesis were included into the study and divided into two groups depending on presence of IHD. Methods included Holter ECG monitoring with analysis of HRV, 2-dimensional and Doppler echocardiography, and bicycle exercise test. The function of heart conduction system and atrium vulnerability (AV) were explored by transesophageal electrophysiological examination. AV was evaluated by AF inducibility, and AF frequency point was estimated. Significant differences have been revealed in two groups regarding size of left atrium (LA), the thickness of ventricular septum and posterior wall of left ventricle (LV). Significant increase of A and decrease of E/A ratio (0.98±0.04 and 1.29±0.07; P<0.001) were identified in patients with PAF associated with IHD. Holter ECG showed significant differences of SDNNi (62.84±2.12 and 74.05±4.42 ms; P<0.05). In conclusion, PAF in patients with IHD were associated with LA dilatation, LV diastolic dysfunction and hypertrophy, HRV decrease, AV increase. Electrical instability in patients without IHD was mainly due to left atrium dilatation and manifested by significant augmentation of supraventricular extrasystoles frequency and AF paroxysms duration measured by Holter ECG. Thromboxane B2 participation in the development of atrial electrical instability was revealed.