Measurement of heart rate variability (HRV) is a non-invasive technique that can be used to investigate functioning of the autonomic nervous system, especially the balance between sympathetic and vagal activities. It is reported that dilatation of coronary microcirculation by augmentation of sympathetic nerve activity (SNA) caused by cold exposure was impaired in diabetes. The question of whether or not SNA in HRV could respond to coronary ischemia was evaluated by cold exposure in diabetic rats. It was found that diabetes with weight loss significantly increased SNA both in baseline and cold exposure, compared with control. A correspondence was also found with coronary ischemia. It can be concluded that measurement of HRV may provide useful information regarding the coronary risk of cold exposure in diabetes. (English translation of J Jpn Coll Angiol 2012; 52: 295-301).