Diabetes mellitus (DM) is clinically associated with an increased incidence of atrial fibrillation (AF), but the underlying mechanism remains unclear. We hypothesized that neural remodeling enhances AF vulnerability in diabetic hearts. Eight weeks after creating streptozotocin-induced diabetic rats (DM rats) or control rats, the hearts were perfused according to the Langendorff method. Inducibility of AF was evaluated by 5 times burst pacing from the right atrium and the atrial effective refractory period (AERP) was measured. The protocol was repeated during sympathetic nerve stimulation (SNS) or parasympathetic nerve stimulation (PNS). In tissue samples taken from the right atrium, the density of nerves positive for tyrosine hydroxylase (TH) and acetylcholinesterase (AChE) were determined. SNS significantly increased the incidence of AF in DM rats (14 ± 6 to 30 ± 8%, P < 0.01), but not in control rats (11 ± 4 to 14 ± 6%, NS). Although AERP was significantly decreased by SNS in both rats (each P < 0.01), increased heterogeneity of AERP by SNS was seen only in DM rats. PNS significantly decreased AERP and increased the incidence of AF (9 ± 5 to 30 ± 5% in control rats, 12 ± 6 to 27 ± 6% in DM rats, each P < 0.01) in both rats. The density of TH-positive nerves was heterogeneous in DM rats compared with control rats, whereas the heterogeneity of AChE-positive nerves was not different in the rats. The prevalence of AF was enhanced by adrenergic activation in diabetic hearts, in which heterogeneous sympathetic innervation was evident. These results suggest that neural remodeling may play a crucial role for increased AF vulnerability in DM.