It was studied a comparative analysis of variability of cardiac rhythm in 105 patients with complicated and uncomplicated arterial hypertension. It was shown that in the majority of patients with stable course of arterial hypertension it was found a moderate reduction in the indices of heart rate variability, reflecting a relative increase of the tone of the sympathetic nervous system while preserving parasympathetic influences. The occurrence of hypertensive crises was associated with a significant decrease in all indices of heart rate variability, clear predominance of sympathetic activity and a sharp oppression inhibition of vagal effects on the cardiovascular system.