Keishi-bukuryo-gan (Gui-zhi-fu-ling-wan) is a formula used for the improvement of blood circulation. Recently it has often also been used for arteriosclerosis. One of the mechanisms involved is thought to be the improvement of endothelial dysfunction, but the details are still unclear. In this study, the effect of Keishi-bukuryo-gan on vascular function and hemorheological factors in spontaneously diabetic (WBN/kob) rats was studied. Rats were given Keishi-bukuryo-gan in chow for 30 weeks. Body weight, blood glucose, endothelium-dependent/-independent relaxation, vasocontraction by free radical-induced and contractive prostanoids, triglyceride, advanced glycation endproduct, lipid peroxides, serum NO2-/NO3- and blood viscosity were measured. The results indicated that Keishi-bukuryo-gan caused a decrease in endothelium-dependent relaxation by acetylcholine to become significantly increased, and vasocontraction induced by free radicals and contractive prostanoids was significantly decreased. Furthermore, serum NO2-/NO3- and blood viscosity were significantly decreased. From these results, it was supposed that Keishi-bukuryo-gan exerted a protective effect on the endothelium. The WBN/kob rat is a useful study model for the complications of human diabetes, and Keishi-bukuryo-gan showed a protective effect against vascular injury in the susceptible rat.