Measurement of anterior cerebral artery blood flow velocity with a continuous wave bidirectional Doppler was compared with cerebral blood flow (CBF) measured with radioactive microspheres in 11 paralyzed newborn lambs during hypoxic hypoxia. The Doppler probe was maintained in a fixed position during each experiment. The objectives of the study were to validate this noninvasive technique that is being used widely in the clinical setting to qualitatively assess changes in CBF, and to evaluate which of the velocity parameters measured provide the most information. Diastolic velocity (DV), peak systolic velocity (PSV), area under the velocity curve (AUC), and pulsatility index (PI) were examined under conditions of varied arterial oxygen content and compared to microsphere CBF. DV (r=0.72, p<0.001), AUC (r=0.72,p<0.001), and PSV (r=0.63,p<0.001) demonstrated stronger correlations with changes in CBF than did the PI (r=-0.41, p<0.05). DV (r=0.81, p<0.001), AUC (r=0.80, p<0.001), and PSV (r=0.75,p<0.001) also exhibited stronger relationships with changes in arterial oxygen content than did the PI (r=–036,p<0.05). These data demonstrate that changes in cerebral blood flow velocity are useful qualitative measures of changes in cerebral blood flow. However, the utility of this technique is dependent upon a stable probe position, and assessment of the actual velocity measurements (DV, PSV, AUC) rather than simply the pulsatility index.