The changes of arterial carbon dioxide partial pressure considerably influence cerebral blood flow and different anesthetic agents have different effects on cerebrovascular physiology. However the importance of these differences in neuroanesthetic practice are unclear. Transcranial Doppler ultrasonography allows the noninvasive direct measurements of cerebral blood flow velocity and direction in the basal brain arteries. The authors performed transcranial Doppler ultrasonography to measure the blood flow velocity of middle cerebral artery in 12 patients who were anesthetized with 10 mcg/kg of fentanyl and 66 % nitrous oxide in oxygen(fentanyl group) and 12 patients with 1.0 vo1% isoflurane and 66 % nitrous oxide in oxygen(isoflurane group) during normocapnia(P(ET)CO(2)=38 mmHg) and hypocapnia(P(ET)CO(2)=28 mmHg)state. The carbon dioxide reactivity was expressed as the changes in mean blood flow velocity per unit changes in endtidal carbon dioxide partial pressure(P(ET)CO(2)). Mean blood flow velocity of middle cerebral artery decreased from 46.6±8.9 cm/s to 30.0±5.3 cm/s in the fentanyl group and 42.7±5.6 cm/s to 32.5±4.6 cm/s in the isoflurane group as the P(ET)CO(2) decreased from 38 mmHg to 28 mmHg. There was a significant difference between the CO2 reactivity of fentanyl group(1.7±0.7 cm/s/mmHg) and isoflurane group(1,0±0,2 cm/s/mmHg) (p<0.05). It is concluded that hyperventilation is more likely to affect cerebral blood flow during fentanyl-nitrous oxide anesthesia than during isoflurane-nitrous oxide anesthesia.