To investigate the effect of acute changes in barometric pressure on regional cerebral perfusion we studied the middle cerebral artery (MCA) blood velocity in five healthy male volunteers by means of a low-pressure chamber. The MCA blood velocity, arterial blood and respiratory gases were measured at the barometric pressures of 1, 0.8, 0.65, and 0.5 atmospheres. The observed blood velocity (Vo) showed no systematic changes. Decreases in barometric pressure induced hypoxia and hypocapnia. When normalizing the MCA blood velocity (Vn) to a standard P(CO2) (5.3 kPa), thereby correcting for the hypoxic induced hypocapnia, we obtained an inverse relationship between cerebral artery blood velocity and arterial blood oxygen content (CaO2). The oxygen supply to the brain, estimated as the product of Vo and CaO2, decreased with lowering of the barometric pressure. However, the product of Vn and CaO2 remained constant. This suggests the existence of a regulatory mechanism attempting to maintain a constant oxygen supply to the brain during acute changes in CaO2, if the hyperventilation induced decrease in PCO2 can be omitted. In the artificial situation of a low pressure chamber, our findings are quite similar to those obtained at sea level. This indicates that the underlying mechanisms of control of cerebral blood flow do not change during acute exposure to altitude.