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To determine the nature and extent of fetal metabolic changes during administration of inhalation anesthesia to the mother, the authors studied maternal and fetal cardiovascular and acid-base changes during general anesthesia with halothane and isoflurane in 16 near-term pregnant ewes with chronically implanted intravascular catheters and uterine-artery flow probes. Three levels of anesthesia were examined for 90 minutes each. During light and moderately deep anesthesia (1.0 and 1.5 MAC) with either agent, maternal blood pressure was slightly depressed (<20 per cent from control), but uterine vasodilatation occurred and uteroplacental blood flow was maintained. Fetal hypoxemia or metabolic acidosis did not occur. Deep levels of anesthesia (2.0 MAC) produced greater reductions (>35 per cent) in maternal blood pressure and cardiac output. Despite uterine vasodilatation, uterine blood flow decreased and the fetuses became hypoxic and acidotic.