To determine whether common carotid artery blood flow measured with an ultrasonic flow transducer would predict brain blood flow in fetal sheep, we measured unilateral common carotid artery blood flow and compared this to simultaneous measurements of total brain blood flows made by radioisotope-labelled microsphere techniques. We studied anaesthetized, exteriorized fetal sheep with intact umbilical circulation after ligation of extracranial, extracerebral arteries and placement of a common carotid artery flow transducer; five fetuses at 120 d gestation had 19 total comparison measurements. As measured by microsphere technique, mean basal blood flow during undisturbed conditions to regional brain areas were similar to normal values reported for the exteriorized ovine fetus; these flows were highly correlated to fetal PaCO2 and successfully varied over a wide range (total brain 9.1-200.4 ml/min/100g and total cortex 6.1-153.1 ml/min/100g) in subsequent experimental conditions of hypercapnia or occluded blood flow. Blood flow as measured by flow transducer significantly correlated (P < or = 0.01) with microsphere measurements of blood flow to total brain (r = 0.56) and total cortex (r = 0.62); regional flow to cerebellum (r = 0.70) and thalamus (r = 0.60) also correlated to transducer measurements. Stronger correlations were observed at low-flow conditions to total brain (r = 0.83) and to total cortex (r = 0.90). As measured by microsphere technique, right and left cortical blood flows were highly correlated (P = 0.0001, r = 0.97), indicating that the flow transducer or surgical manipulation did not disturb the distribution of cerebral blood flow. The mean values for zero flow reference of the transducer were < 1.5% of mean basal flow values. It is concluded that the common carotid artery flow transducer technique developed in this study provides an accurate prediction of blood flow to total brain and total cortex over a wide range of values in fetal sheep. This technique provides a methodologic advantage to sequential experimental interventions and may prove advantageous to studies of fetal sheep cerebral circulation.