The authors studied a series of 87 blood flow measurements in the internal carotid arteries of fetuses. After a brief survey of cerebral blood flow a description is given of the technique for measuring this flow. They use the index D/S just as in measuring umbilical artery flow. In normal pregnancy the carotid diastolic index hardly rises and stays low, which confirms that there is a high resistance in the system. When abnormal flow rates are found in the umbilical artery the carotid diastolic index rises to protect the brain against hypoxia. The rise in the carotid diastolic index could be the last abnormality which precedes the appearance of fetal cardiac rhythm changes.