The heart rate (HR) variation of 25 normotensive and asymptomatic men, mean age 58 +/- 7 years, with diabetes mellitus (group I) was studied during deep respiration. Thirteen subjects (52%) had a variation of 10 beats/min or less, consistent with an autonomic neuropathy (AN) (group IA); 12 had variation in HR of more than 10 beats/min and were considered to have no neuropathy (group IB). The 24-hour ambulatory HR and systolic blood pressure (BP) values of group I were compared with those of 13 healthy men, mean age 48 +/- 8 years (group II). The mean of 5 maximal HR measurements during the 24-hour period was higher for group IA (106 +/- 11 beats/min) than for group IB (100 +/- 13 beats/min) or for group II (92 +/- 9 beats/min) (p less than 0.01). The mean of 5 maximal BP measurements was greater for group I (149 +/- 28 mm Hg) than for group II (128 +/- 13 mm Hg) (p less than 0.01), but no difference was observed between groups IA and IB. Maximal treadmill exercise was performed with 22 of the patients (11 with and 11 without AN), and no difference in HR was observed between the 2 groups during all stages of exercise or at maximal exertion. The increase in systolic BP and duration of exercise in these 2 groups were also similar. Seventeen of 25 diabetic men had peripheral neuropathy (PN). Of 13 patients with AN, 10 had PN; of 12 without AN, 7 had PN and 5 did not.(ABSTRACT TRUNCATED AT 250 WORDS)