The study was designed to investigate the effects of MokhwyangJoki-san Extract (MJSE) on the change of regional cerebral blood flow (rCBF) and mean arterial blood pressure (MABP) in normal rats, and further to determine the mechanism of MJSE. The results in normal rats were as follows ; MJSE significantly increased rCBF in a dose-dependent manner, and MABP did not change in a dose-dependent manner. This results were suggested that MJSE significantly increased rCBF by dilating pial arterial diameter. The MJSE-induced increase in rCBF was significantly inhibited by pretreatment with methylene blue (10 ${mu}g/kg$, i.p.), an inhibitor of guanylate cyclase, and was not changed by indomethacin (1 ${mu}g/kg$, i.p.), an inhibitro of cyclooxygenase. The MJSE-was not changed MABP was decreased by pretreatment with indomethacin but was not changed by methylene blue. This results were suggested that the mechanism of MJSE was mediated by guanylate cyclase.