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Extract: Cerebral blood flow (CBF) with the 133Xe clearance technique and cerebral oxygen consumption were measured in the fetal lamb using chioralose-anesthetized ewes and acutely exteriorized fetuses with intact umbilical circulation. To induce fetal hypoxia at different pH levels, three different procedures were used: (1) the ewe was ventilated with a hypoxic gas mixture, containing 8–15% O2in N2; (2) 5–10% CO2was added to the hypoxia gas mixture; (3) during hypoxia a continuous rapid infusion of NaHCO3solution was given intravenously to the fetus. Blood gas tensions, pH, and oxygen saturation were measured repeatedly.Multiple regression analysis was carried out on the data with the variables SaO2, PaO, PaO2, and pH considered as independent variables and flow as the dependent variable. All complete sets of observations were used amounting to 74 observations in 11 animals. No significant correlation was found between CBF and pH. SaO2showed the highest correlation to CBF. No further improvement of this correlation was obtained when PaCO2was added as independent variable. PaO2and PaCO2described jointly the variations of CBF as well as SaO2alone.The oxygen consumption of the brain decreased during hypoxia when SaO2was reduced below 40%. The decrease of the metabolic rate for oxygen was a function both of the degree of hypoxia and of pH. When an acidosis was added to the hypoxia a significant reduction of the oxygen consumption of the brain resulted compared with the same degree of hypoxia at normal or only moderately reduced pH levels.Speculation: The cerebral blood flow of the fetal lamb is regulated mainly via the amount of oxygen available to the brain, and changes of CO2tension affect CBF via a displacement of the hemoglobin dissociation curve.Cerebral oxygen consumption is reduced to dangerously low levels when a severe acidosis is combined with hypoxia because of a failure to extract the oxygen available in the arterial blood.