We have determined in 46 control subjects and 117 diabetic patients with or without peripheral neuropathy (PN) the blood pressure response to standing and the heart rate variations during deep breathing (HRV) and standing (T/B ratio). HRV was abnormal in 20% of diabetics and T/B ratio only in 8%. HRV and T/B ratio decreased with age both in control and diabetic group and significant alterations of these two tests in diabetic patients were found only for the youngest subjects; HRV and T/B ratio were not related to the duration of diabetes. Ten diabetics had orthostatic hypotension. Diabetics with PN had lower HRV and T/B ratio than the others and, excluding the 10 subjects with large orthostatic hypotension, a lack of increase in diastolic blood pressure and a transient fall in systolic blood pressure on standing. T/B ratio appears in our subjects of less value than HRV for the diagnosis of cardiac autonomic neuropathy; because of the spontaneous alterations with age of the response to these tests interpretation of the results in diabetics is possible only in the presence of appropriate data in control subjects. PN in diabetic patients is strongly associated with abnormalities of heart rate regulation suggestive of cardiac vagal neuropathy and with abnormalities of blood pressure regulation suggestive of peripheral sympathetic lesions.