Autonomic neuropathy is a common and severe complication of diabetes mellitus that leads to dysfunction of the cardiovascular system. The reduced ability to finely regulate heart rate is attributed to an impairment of cardiac parasympathetic regulation, but it is not known whether this is due to parasympathetic neuropathy and/or direct cardiac impairments. Therefore, we recorded the electrocardiogram of streptozotocin-induced diabetic rats under basal conditions and during electrical stimulation of the vagus nerve. We used the neurotrophic agent Org 2766, an adrenocorticotropic hormone [ACTH]-(4-9) analogue, to investigate the involvement of a neurogenic component in the altered vagal control of heart rate. The R-R interval was increased and atrioventricular transmission time unchanged 1 week after diabetes induction and remained so until 20 weeks. Treatment with Org 2766 could not prevent the bradycardia. After bilateral vagotomy, both diabetic and non-diabetic rats had the same R-R and P-R interval. The response of the R-R interval to electrical stimulation of the right vagus nerve was impaired, and this impairment was not reversed by Org 2766 in diabetic rats. These results suggest that neurogenic factors are of little or no importance in the impaired parasympathetic control of heart rate seen in experimental diabetes.