The effect of moderate hypoxia on cerebral blood flow (CBF) in man has not been well described, and little is known about the interaction of changes in arterial PO2 and PCO2 as regards CBF. Using a non-invasive doppler ultrasound method we have measured the instantaneous mean blood velocity (which is proportional to CBF as long as the cross-section of the vessel is constant) in the carotid artery in four healthy unanaesthetized subjects. We found in all subjects that a reduction in alveolar PO2 from about 13 to about 8.7 kPa with maintained constant alveolar PCO2 (PA, CO2) caused CBF to increase gradually over 10 min (half-time about 4 min) to about 125% of control. The CBF decreased quickly (half-time about 45 s) towards control when alveolar PO2 was reset to 13 kPa. As measured 5 min after a step-change in PA, O2, the change in CBF was independent of PA, CO2 within the range 3.3-6.7 kPa. An increase in PA, O2 to about 33 kPa reduced CBF only if PA, CO2 was in the hypercapnic range. Unexpectedly we found that the CBF response showed 'adaptation' during both maintained increase and decrease in PA, CO2. The CBF started to return towards control level within 10 min after induction of hypo- or hypercapnia. We conclude that also moderate hypoxia causes increased CBF in unanaesthetized man within a wide range of PA, CO2.