In 18 spontaneously breathing, preterm infants (mean gestational age 30.3 weeks) cerebral blood flow (CBF) was investigated twice, 2 and 3 hours after birth when spontaneous changes in arterial carbon dioxide tension (PaCO2) and mean arterial blood pressure (MABP) were expected. Transcutaneous oxygen tension (TcO2) was kept normal by adjusting the inspiratory oxygen fraction. In 12 infants, plasma adrenaline and noradrenaline were constant throughout the study. Changes in CBF infinity (CBF infinity) were significantly related to changes in PaCO2 (p = 0.0001) whereas neither changes in MABP nor TcO2 reached a significant association to changes in CBF infinity (p = 0.67 and p = 1.0, respectively). The calculated CBF infinity -CO2 reactivity of 28.9% per kPa PaCO2 (95% confidence interval 16.1-43.0) is comparable to findings in older newborns and healthy adults. Only one of 18 infants developed germinal layer haemorrhage (grade I) in spite of the hypercapnic state which was observed during the first hours of life. Periventricular leucomalacia was not detected. It is suggested that the cerebral blood flow is well regulated within physiological variations of PaCO2 and MABP in the healthy, preterm newborn even shortly after birth.