To reveal the influence of therapeutically induced changes of arterial blood pressure on cerebral circulation, pulsed Doppler measurements of blood velocity in the right internal carotid artery were performed in 23 neonates. A positive correlation between mean arterial blood pressure and time-averaged maximum blood velocity (change more than 0.5%/torr) could be noticed in 16 infants. These infants were supposed to have loss of autoregulation. The main characteristics in this non-autoregulating group were: gestational age less than 31 weeks, birth weight less than 1501 g and mean carotid blood velocity less than 20 cm/s. In accordance with animal experiments we assume that autoregulation does not work below a definite lower limit of brain perfusion, which is reflected by carotid blood velocity in our study. Patients below/equal 1500 g or 30 gestational weeks very often do not exceed this limit and thus do not reach the "range of autoregulation".