We evaluated the effects of N-hexacosanol, a cyclohexenonic long-chain fatty alcohol, on muscarinic receptors in diabetic rat ileal dysfunction. Eight-week-old male SD rats were divided into four groups. After induction of diabetes (streptozotocin 50 mg/kg, i.p.), three groups were maintained for eight weeks with treatment by N-hexacosanol (0, 2 or 8 mg/kg, s.c. every day). Ileum function was investigated by organ bath studies using carbachol and KCl, and the expression levels of muscarinic M 2 and M 3 receptors were investigated by real-time polymerase chain reaction. Various concentrations of subtype-selective muscarinic antagonists, i.e., atropine (non-selective), pirenzepine (M 1 selective), methoctramine (M 2 selective), and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, M 1 /M 3 selective), were used in this study. In the presence and absence of these antagonists, contractile response curves to increasing concentrations of carbachol were investigated. Treatment with N-hexacosanol did not alter the diabetic status of the rats, but did significantly prevent the carbachol-induced hypercontractility in diabetic rat ileum. Estimation of the pA 2 values for atropine, pirenzepine, methoctramine, and 4-DAMP indicated that the carbacholinduced contractile response in the ileum is mainly mediated through the muscarinic M 3 receptor subtype in all groups. Furthermore, N-hexacosanol significantly prevented the diabetes-induced up-regulation of intestinal muscarinic M 2 and M 3 receptor mRNAs in streptozotocin-diabetic rats. Our data indicated that N-hexacosanol exerts preventive effects with respect to carbachol-induced hypercontractility in the diabetic rat ileum without qualitative alteration of the muscarinic receptor system.Gastrointestinal smooth muscles receive a variety of excitatory and inhibitory inputs from the enteric nervous system. As cholinergic nerves are activated, the neurotransmitter acetylcholine (ACh) is released from their terminals. Subsequently, ACh acts on smooth muscle cells to activate cell-surface muscarinic receptors, thus activating various intracellular signaling pathways, and in turn inducing smooth muscle contraction (6, 7). To date, five muscarinic receptor subtypes (M 1 -M 5 ) have been identified (5,20). A recent reverse transcriptase-polymerase chain reaction (RT-PCR) study reported the possible expression of all five subtypes in gastrointestinal smooth muscle (5). The muscarinic M 2 and M 3 receptor subtypes are preferentially expressed, with a preponderance of the former subtype. In general, the muscarinic M 3 receptor subtype in smooth muscle is coupled to the G protein Gq/G11 and mediates the