Heart rate variability (HRV) was assessed on 24 hour Holter ECG recordings from 32 healthy persons and 54 patients with paroxysmal atrial fibrillation (AF) of different etiology (hypertension, ischemic heart disease, rheumatic mitral valve disease, idiopathic AF) aged 45-73 years. Nocturnal, diurnal and 24 hour HRV was measured. Time and frequency domain HRV measures were lower in patients with AF and underlying heart disease compared with healthy persons and for most of the parameters with patients with idiopathic AF (p<0,05). Patients with idiopathic AF compared with healthy persons had higher values of high frequency power during nighttime (p<0.05), what reflected activation of parasympathetic nervous system in this category of patients.