Geopungdodam-tang(GDT) have been used in oriental medicine for many centuries as a therapeutic agent of apoplexy. The mechanism of GDT on the cerebral blood flow is not known. The purpose of this Study was to investigate effects of GDT on the pial arterial diameter and action mechanism of GDT-induced increased regional cerebral blood flow(rCBF). The changes of regional cerebral blood f1ow(rCBF) was determinated by Laser-Doppler Flowmetry(LDF), and the changes of pial arterial diametet were determinated by video microscopy methods and video analyzer. The results were as follows ; 1. Pial arterial diameter was significantly increased by GDT in a dose-dependent manner. 2. Pretreatment with L-NNA significantly inhibited GDT induced increased rCBF. 3. Pretreatment with methylene blue significantly inhibited GDT induced increased rCBF. 4. Pretreatment with indomethacin inhibited GDT induced increased rCBF. These results suggest that GDT causes a diverse response of cerebral hemodynamics(rCBF and pial arterial diameter). The cerebral hemodynamics is also mediated by nitric oxide synthase, cyclic GMP(guanylyl cydase) and prostaglandin(cyclooxygenase).