Reduced cardiac parasympathetic activity is involved in sudden cardiac death and is responsible for high mortality in patients with type 2 diabetes mellitus (T2DM). Nicotinic acetylcholine receptors (nAChRs) mediate synaptic transmission in the intracardiac ganglia (ICG) and regulate the excitability of postganglionic parasympathetic neurons. We hypothesized that changes in the functional responsiveness of postsynaptic nAChRs to nicotine (a nAChR agonist) might be involved in attenuated cardiac parasympathetic (vagal) activity in type 2 diabetic rats. A rat model of T2DM was induced by a combination of both high-fat diet and injection of low-dose streptozotocin (30 mg/kg, i.p.). As an index of cardiac vagal function, changes of heart rate in response to graded vagal efferent nerve stimulation were blunted in T2DM rats, compared to sham rats. In isolated Langendorff-perfused hearts, there was no significant difference on sensitivity of the heart to acetylcholine in sham and T2DM rats. Whole-cell patch-clamp data showed that T2DM decreased nAChR currents and sensitivity of the nAChR channel to nicotine in ICG neurons. However, the data from immunofluorescence staining showed that there is no significant difference in the protein expression of ?3 and ?4 nAChR subunits in ICG neurons of sham and T2DM rats. These results suggest that the low sensitivity of nAChR channels to nicotine might contribute to impairment of the cardiac vagal activity in T2DM.