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Extract: Somatosensory evoked electroencephalogram (EEG) responses (SER) were recorded in 19 fetal lambs with gestational ages varying from 66 to 140 days. The ewes were anesthetized with chloralose and the fetuses were exteriorized with intact umbilical circulation. Periods of hypoxia at different pH levels were induced by ventilating the ewe with gas mixtures including 8–15% oxygen, either alone or combined with a 10% CO2mixture, or with an intravenous infusion of bicarbonate to the fetus.The reaction of the SER during hypoxia followed two main patterns, one seen in the immature fetus and one in the more mature fetus. In the immature type of response (seen in all fetuses at or less than 110 days of gestational age) the primary response of the SER is more sensitive to oxygen lack than the secondary components, whereas in the mature type of response (seen at or more than 115 days of gestational age) the reverse is true.At moderate degrees of hypoxia, PaO2between 15 and 20 mm Hg, the SER were moderately affected provided that the pH of the blood was within normal limits. At lower PaO2, values, and when moderate hypoxia was combined with acidosis, the SER were seriously affected or completely abolished. At PaO2, values above 20 mm Hg an acidosis had no effect on the SER.Speculation: The function of the fetal brain is seriously jeopardized by the combination of hypoxia and acidosis. Therefore, both hypoxia and acidosis should be counteracted during asphyxia to prevent brain damage.