BACKGROUND AND PURPOSE: Cerebrovascular reactivity (CVR) can be estimated by measuring the change of cerebral blood flow that occurs during vasostimulation. To estimate the cerebrovascular reactivity, we investigated the change of flow velocity of the internal carotid artery (ICA) and the middle cerebral artery (MCA) during hyperventilation and hypoventilation with the transcranial doppler. So we studied whether the CVR measured by this method could show a significant difference between the normal and the atherosclerotic subjects and whether the CVR may decrease with age in normal gubjects. METHODS: Using transcranial doppler, we measured the mean velocity (Vm), the pulsatility index (P.I.) at the resting state, the end of breath-holding and the end of hyperventilation in 30 normal and 10 atherosclerotic subjects, so we calculated the percentile change of mean velocity (% Vm) and P.I. (% P.I.) after the vasostimulation. We estimated the change of Vm, P.I., % Vm and % P.I. By the age group and compared those parameters between the age-matched normal control and atherosclerotic subjects. RESULTS: The Vm in ICA and MI significantly decreased with age (p<0.01), but there was no significant difference in Vm and P. I. Between normal and atherosclerotic subjects. The % Vm and % P.I. In response to hyperventilation significantly decreased with age in ICA, M1, M2 and there was significant difference in % Vm of ICA and M1 after breath-holding and % Vm of ICA after hyperventilation between the normal and atherosclerotic subjects. CONCLUSION: The breath-holding and hyperventilation tests could be non-invasive and useful methods in estimation of the cerebrovascular reactivity and could be applied in the basal and follow-up evaluation of the cerebrovascular reserve of the ischemic stroke patients.