Circadian dynamics of heart rate variability (HRV) was studied in patients with ischemic heart disease (IHD) with paroxysmal atrial fibrillation, with IHD and stable angina without clinically significant arrhythmia and healthy subjects of the same age. Special emphasis was placed on spontaneous changes of HRV parameters on short term ECG recordings obtained during unrestricted activity of a patient and dynamics of HRV parameters preceding nocturnal attacks of atrial fibrillation. The findings were decrease of total HRV, lowering throughout 24 hours of low and very low frequency power, and absence of circadian dynamics of characteristics of high frequency component of variability. HRV changes preceding attacks of atrial fibrillation were variable and in most cases both low and high frequency components were involved. This could evidence for participation in triggering of atrial fibrillation of both sympathetic and parasympathetic influences.