Heart rate variation was measured at different rates of regular deep breathing (2.5 to 13 breaths/min) in 11 normal subjects and 13 diabetic patients with neuropathy, defined clinically by the absence of both ankle jerks or the presence of autonomic symptoms. Subjects were divided into those above and below 30 years old. Normal subjects showed twofold changes in heart rate variation with maximum variation at mean breathing rates of 6.3 and 5.5 breaths/min. Heart rate variation was significantly smaller than normal in the diabetic patients with neuropathy, at all of the frequencies tested in the younger group and 7 of the 13 test frequencies in the older group; the maximum variation occurred at slower mean breathing rates of 4.5 breaths/min (p less than 0.005) and 4.1 breaths/min (p less than 0.01) respectively. Seven diabetic patients without neuropathy, with intact leg reflexes and less than 30 years old, had normal heart rate variation but the maximum responses occurred at a mean breathing rate of 5.4 breaths/min, significantly slower than normal (p less than 0.02). Measuring the breathing rate at which heart rate variation is maximal may be a sensitive way of detecting autonomic (vagal) neuropathy.