OBJECTIVE: Decreased heart rate variability (HRV) is associated with enhanced mortality due to abnormal cardiac rhythm. While hypoglycemic events are increasingly common in the treatment of type 2 diabetes, HRV is part of the counter-regulation against low blood glucose levels. We hypothesized that HRV was impaired in mild hypoglycemia in diabetic individuals. MATERIALS/METHODS: Hyperinsulinemic-hypoglycemic clamps were performed in twelve type 2 diabetic patients without cardiovascular disease and in non-diabetic subjects matched for age, sex, and weight. In an additional study, hypoglycemic events, induced by either a single morning dose of glibenclamide or physical exercise, were recorded for the subsequent 24h. Blood glucose concentrations and electrocardiograms were continuously monitored. Serum hormone levels, hypoglycemic symptoms, and forearm blood flow were measured at defined time points. RESULTS: Occurrence of a symptomatic hypoglycemic episode (mean blood glucose 3.1+/-0.4 mmol/l) attenuated most of the time and frequency domain measurements in both healthy and diabetic individuals. The magnitude of reduction of HRV parameters was significantly lower in diabetic compared to healthy subjects. Glibenclamide taken in the morning enhanced the daily number of mild hypoglycemic events compared with placebo or moderate exercise. Concordantly, 24-h mean HRV measurements were decreased. CONCLUSION: HRV response to hypoglycemia is impaired in type 2 diabetic subjects resulting in a higher than expected risk for sudden arrhythmia following mild hypoglycemic episodes.