The CBF was measured on the first three days of life in 22 mechanically ventilated, preterm neonates (mean gestational age 29.5 weeks) with simultaneous recordings of arterial carbon dioxide tension (PaCO2), arterial oxygen tension (PaO2), haemoglobin concentration (Hgb), haemoglobin oxygen saturation and mean arterial blood pressure (MABP). CBF infinity tended to increase slightly with age. The intra-individual variation of CBF infinity was positively related to changes in PaCO2 (p = 0.0004) and inversely related to changes in Hgb (p = 0.029). Neither PaO2 nor MABP achieved a significant relation to changes in CBF infinity. Thus, the mean CBF infinity -CO2 reactivity was calculated to 22.1% per kPa whereas CBF infinity increased by a mean of 11.9% per mM decrease of Hgb thereby providing a constant oxygen delivery to the brain. It is concluded that PaCO2 and Hgb have the expected effect on CBF infinity in preterm neonates even during respiratory distress shortly after birth.