In epineurial arterioles, acetylcholine-mediated vascular relaxation is mediated by nitric oxide and endotheliumderived hyperpolarizing factor (EDHF), and both mechanisms are impaired by diabetes. The mediator responsible for the effect of EDHF is unknown. In epineurial arterioles, C-type natriuretic peptide (CNP) has properties consistent with EDHF-like activity. Epineurial arterioles express CNP, and exogenous CNP causes a concentration-dependent vascular relaxation. In streptozotocin-induced diabetic rats, CNP-mediated vascular relaxation in epineurial arterioles is decreased. Since CNP may be a regulator of vascular function, a vasopeptidase inhibitor may be an effective treatment for diabetes-induced vascular and neural disease. Vasopeptidase inhibitors inhibit ACE activity and neutral endopeptidase, which degrades natriuretic peptides. Streptozotocin-induced diabetic rats were treated with AVE7688 (450 mg/kg in the diet), a vasopeptidase inhibitor, for 8 -10 weeks after 4 weeks of untreated diabetes. Treatment of diabetic rats corrected the diabetesinduced decrease in endoneurial blood flow, significantly improved motor and sensory nerve conduction velocity, prevented the development of hypoalgesia in the hind paw, and reduced superoxide and nitrotyrosine levels in epineurial arterioles. The diabetes-induced decrease in acetylcholine-mediated vascular relaxation by epineurial arterioles was significantly improved with treatment. These studies suggest that vasopeptidase inhibitors may be an effective approach for the treatment of diabetic vascular and neural dysfunction. Diabetes 56:355-362, 2007 S tudies performed in our laboratory with streptozotocin-induced diabetic rats have provided evidence that the generation of oxidative stress through the production of superoxide and peroxynitrite impairs vascular function of epineurial arterioles of the sciatic nerve, and this precedes slowing of motor nerve conduction velocity (MNCV) (1-3). These studies imply that hyperglycemia-induced oxidative stress may be responsible for diabetes-induced vascular and neural dysfunction that occurs during diabetic neuropathy.We have recently reported that the ACE inhibitor Enalapril was an effective treatment for vascular and neural disease in streptozotocin-induced diabetic rats (4). These results and our studies with C-type natriuretic peptide (CNP), demonstrating its vascular relaxation properties in epineurial arterioles, provide rationale to test the effect of vasopeptidase inhibitors on diabetic neuropathy.It has been reported in mesenteric resistance arteries that release of CNP accounts for the biological activity of endothelium-derived hyperpolarizing factor (EDHF) (5,6). CNP was found to be released from endothelial cells of the perfused rat mesenteric bed in response to endotheliumdependent vasodilators such as acetylcholine (5,6). CNP induced hyperpolarization and relaxation of mesenteric artery vascular smooth muscle through activation of natriuretic peptide receptor subtype B and the same potassium channels that...