Therefore, the present study was to investigate the effect of PDTC on impairment of endothelium-dependent relaxation and vascular DDAH activity in streptozotocin (STZ)-induced diabetic rats, and to further compare it with the effect of ex vivo DDAH transfection, so as to explore the underlying mechanism by which PDTC ameliorates diabetes-induced impairment of endothelium-dependent vasodilation. This evidence concerns the gene DDAH1 and diabetes mellitus.