Decreased arterial carbon dioxide tension (PaCO2) results in decreased cerebra l blood flow, which is associated with diminished cerebral electrical activity. In such a situation, cerebral fractional oxygen extraction (CFOE) would be expec ted to increase to preserve cerebral oxygen delivery. This study aimed to determ ine whether changes in blood gases in infants less than 30 wk’gestation were as sociated with changes in background electroencephalograms (EEG) and CFOE. Thirty -two very low birth weight infants were studied daily for the first three days after birth. Digital EEG recordings were performed for 75 min each day. CFOE, me an blood pressure and arterial blood gases were measured midway through each rec ording. EEG was analysed by (a) spectral analysis and (b) manual calculation of interburst interval. Blood pressure, pH and PaCO2 did not have any effect on the EEG. On day one, only PaCO2 showed a relationship with the relative power of th e delta frequency band (0.5-3.5 Hz) and the interburst interval. The relative p ower of the delta band remained within normal limits when PaCO2 was between 24 a nd 55 mm Hg on day one. There was a negative association between PaCO2 and CFOE. The associations between PaCO2 and EEG measurements were strongest on day one, weaker on day two, and absent on day three. The slowing of EEG and increased CFO E at lower levels of PaCO2 are likely to be due to decreased cerebral oxygen del ivery induced by hypocarbia. When PaCO2 was higher, there was suppression of the EEG. Decreased arterial carbon dioxide tension (PaCO2) results in decreased cerebra l blood flow, which is associated with diminished cerebral electrical activity. In such a situation, cerebral fractional oxygen extraction (CFOE) would be expec ted to increase to preserve cerebral oxygen delivery. learn aimed to determ ine whether changes in blood gases in infants less than 30 wk’gestation were as sociated with changes in electroencephalograms (EEG) and CFOE. Thirty-two very low birth weight infants were studied daily for the first three days after birth . EEG was analyzed by (a) spectral analysis and (b) manual calculation of interburst interval. Blood pressure, pH and PaCO2 did not have any effect on the EEG. On day one, only PaCO2 showed a relationship with the relative power of th e delta frequency band (0.5-3.5 Hz) and the interburst interval. The relative power of the delta band remained within normal limits when PaCO2 was between 24 a nd 55 mm Hg on day one. There was a negative association between PaCO2 and CFOE. The associations between PaCO2 and EEG measurements were strongest On day one, weaker on day two, and absent on day three. The slowing of EEG and increased CFO E at lower levels of PaCO2 are likely to be due due to cerebral oxygen del ivery induced by hypocarbia. When PaCO2 was higher, there was suppression of the EEG.