It was previously suggested that respiratory heart rate (HR) variations were predominantly affected by the parasympathetic nerve and HR increase to standing was predominantly affected by the sympathetic nerve. To compare parasympathetic and sympathetic nerve function in diabetics, these two tests were performed in 95 diabetics and 38 controls by use of an instantaneous-HR-change continuous recorder. All subjects were between 40 and 59 years-old. As indices of autonomic nerve function, the mean difference between maximal and minimal HR during deep breathing (delta I-E) and the HR increase on standing (delta HR) were determined. The mean delta I-E and delta HR in the diabetics were 9.4 beats/min and 15.1 beats/min, respectively. These values were significantly lower than those in the controls (delta I-E 14.4, delta HR 20.5 beats/min). delta I-E correlated negatively to duration of diabetes and mean fasting blood glucose during the last 6 months in the diabetics, but delta HR did not correlate to them at all. Diminished delta I-E was found in the patients with insulin treatment, retinopathy or persistent proteinuria. Diminished delta HR, however, was found only in the patients with long-standing complicated diabetes. Thus, both cardiac parasympathetic and sympathetic nerve function were significantly impaired in the diabetics as compared with the controls and that parasympathetic nerve damage occurred early whereas sympathetic innervation was preserved.