Because onset of autonomic neural dysfunction in the diabetic syndrome has not been well established, sensitive and quantitative measures of autonomic nervous system (ANS) function were made in 19 non-insulin-dependent (NIDD) and 14 insulin-dependent (IDD) recent-onset diabetic subjects. The known duration of diabetes mellitus in the NIDD subjects was ≤ 12 mo. The duration in the IDD subjects was ≤ 24 mo. RR-variation during beta adrenergic blockade (an index of an ANS reflex involving the cardiac parasympathetic nervous system [PNS] pathway) was smaller than that of control subjects in both NIDD (P < 0.001) and IDD subjects (P < 0.01). This PNS abnormality was not likely to be due to volume depletion since acute volume depletion induced by furosemide in six normal subjects (1608 ± 105 ml, mean ± SEM) did not change RR-variation. Dark-adapted pupil size after topical PNS blockade (an index of iris sympathetic nervous system [SNS] activity) was also smaller in both groups of diabetic subjects (NIDD, P < 0.01; IDD, P < 0.05). Pupillary latency time (an index of an ANS reflex involving iris PNS pathway) was prolonged in the NIDD subjects (P < 0.005) but was not significantly altered in the IDD subjects. Thus, it would appear that the ANS is impaired soon after the diagnosis of diabetes mellitus. We hypothesize that early impairment of the ANS is common in IDD and NIDD subjects. This finding is consistent with the hypothesis that abnormal carbohydrate metabolism is an important factor in the etiology of diabetic autonomic neuropathy.