Objectives : The purpose of this study is to investigate whether Haenggan(LR2) Reduction in Acupuncture affects cerebral hemodynamics〔regional cerebral blood flow(rCBF), mean arterial blood pressure(MABP)〕in normal rats, and to make manifest whether Haenggan(LR2) Reduction in Acupuncture is mediated by cyclooxygenase or guanylate cyclase. Methods : This experiments was to investigate at the other changes of rCBF and MABP at Haenggan(LR2) Reduction in Acupuncture in normal rats, pretreated rats with indomethacin(1 mg/kg, i.v.) and pretreated rats with methylene blue$(10;{,u}g/kg,;i.v.)$. Results : 1. Haenggan(LR2) Reduction in Acupuncture was significantly increased rCBF during acupuncture and after withdrawing of the needle. 2. Haenggan(LR2) Reduction in Acupuncture was decreased MABP during acupuncture, but Haenggan(LR2) Reduction in Acupuncture was increased MABP in compared with normal condition. 3. Pretreatment with indomethacin(1 mg/kg, i.v.) was significantly inhibited Haenggan(LR2) Reduction in Acupuncture induced increase of rCBF, but was increased Haenggan(LR2) Reduction in Acupuncture induced increase of MABP. 4. Pretreatment with methylene blue$(10;{mu}g/kg,;i.v.)$ was significantly decreased Haenggan(LR2) Reduction in Acupuncture induced increase of rCBF and MABP. This results suggest that Haenggan(LR2) Reduction in Acupuncture increased rCBF by dilating pial arterial diameter, and the mechanism of Haenggan(LR2) Reduction in Acupuncture is mediated by guanylate cyclase.