SummaryThis study investigates the possibility of using pulsatile transcephalic impedance changes, ?Z, for the continuous monitoring of cerebral blood flow, CBF, in the sick newborn infant. The performance of the impedance method is compared with the measurement of cranial flow, Fc, using a strain gauge for detecting the predicted changes in cerebral blood flow when the baby breathes 2% CO2in air, or O2.In all five studies in which measurements were compared in air and CO2the expected increases in Fc and ?Z were seen. In six of seven studies comparing measurements in air and O2the expected fall in the variables was seen, indicating reduced cerebral blood flow. The problems of both these methods are discussed and we conclude that the impedance method is as good as the strain gauge method for detecting changes in cerebral blood flow.