Cerebral blood flow measurement has been an important investigative tool in newborns at risk for perinatal brain damage. The validity of Doppler cerebral blood flow velocity measurements depends on a constant vessel diameter. We have validated Doppler against the electromagnetic flowmeter (EM) using a modified common carotid artery model in the 1-d-old piglet. Two sets of continuous and simultaneous recordings were performed:1) the Doppler and EM probe on the same common carotid artery (extracerebral branches were ligated), and2) the EM probe on the common carotid artery and the Doppler probe recording from an intracerebral artery through an artificial fontanel. Arterial partial pressure of carbon dioxide (PaCO2) was manipulated (2.8–7.4 kPa), as was arterial blood pressure (3.7–9.3 kPa). Simultaneous EM flow and estimated Doppler flow were compared. Ninety-three recordings were obtained with both transducers on the modified carotid artery, and 49 were obtained with the Doppler insonicating an intracerebral artery. A multiple regression model was used for statistical analysis. The correlation between EM and both sets of Doppler measurements for individual animals was >0.95 and was unaffected by changes in arterial blood pressure or PaCO2. Thus, the common carotid and the intracerebral artery investigated did not change their diameter significantly in response to PaCO2or arterial blood pressure. The relationships between EM and Doppler in the individual animal were all linear but revealed great variability in the slopes due to the unknown vessel diameter and angle of insonication. We conclude that relative changes of cerebral blood flow velocity in the modified common carotid artery and intracerebral arteries show the same changes as common carotid artery blood flow (determined by an EM) in the healthy 1-d-old piglet.