The sympathetic and parasympathetic systems are integrated and are complementary; both of these take part in the intricate regulation of cardiac functions, ensuring a proper adjustment of the functioning of the heart. In the normal person the most important nervous control of heart is parasympathetic which is mainly active during the calm, steady, and homeostatic state whereas the sympathetic system is concerned with mobilization of energy during emergency and stressful situations. Heart rate variation during deep breathing is another method of evaluating parasympathetic integrity in the diabetic patient. Catecholamine response to exercise is blunted in patients with slight autonomic neuropathy and possibly even more impaired in patients with severe neuropathy. Autonomic dysfunction is a frequent complication of diabetes and both parasympathetic and sympathetic divisions of the autonomic nervous system appear to be involved. Elevated levels of plasma norepinephrine may have some bearing on the pathophysiology of cardiovascular dysfunction in diabetes.