Objectives: Uwhangchungsim-won(UC) has been used in the treatment of a wide variety of conditions including stroke, hypertension, arterosclerosis, autonomic imbalance, and mental instability, in Korean traditional hospitals. The aim of this study was to evaluate the effect of DC on cerebral hemodynamics and to determine the appropriate dosage. Methods: We studied changes in hyperventilation-induced cerebrovascular reactivity and mean blood flow velocity of the middle cerebral arteries(MCAs) were studied by means of transcranial Doppler ultrasound. Changes in mean blood pressure, pulse rate and expiratory CO2(PECO2) were observed using Cardiocap TM/5. Six healthy young volunteers who were administrated with full doses of DC for group A, and half doses for group B. Six other healthy subjects comprised the control group. The evaluation was performed during basal condition, and repeated at 20, 40, 60, 120, and 180 minutes after administration. Results: Increases of cerebrovascular reactivity and mean blood flow velocity in the middle cerebral artery in group A were significantly different compared with group B and the control group (p<0.1). Mean blood pressure, pulse rate and expiratory CO2 did not change during the observation and were not different among these three groups. We observed that in cerebrovascular reactivity induced hyperventilation, group A was most effective at 40 minutes after administration, and its effectiveness lasted for 120 minutes. Conclusions: This study provides evidence for UC, in full doses, as an agent for dilation of the cerebral arteriols to increase hyperventilation-induced cerebrovascular reactivity as a consequence of faster recovery of blood flow velocity.