A combined sector and pulsed Doppler system was used to study the pulsatility index in the fetal internal carotid artery and umbilical artery in 156 normal pregnancies and 42 cases of intrauterine growth retardation (birth weight below the tenth percentile). All pregnancies were in the third trimester. In normal pregnancies, there was a gestational age-related fall in pulsatility index for both the umbilical artery and the umbilical artery/internal carotid artery ratio. No such fall was established for the pulsatility index in the internal carotid artery. In growth-retarded pregnancies, raised pulsatility index values in the umbilical artery were associated with reduced pulsatility index values in the internal carotid artery, suggesting the presence of a "brain-sparing" effect. When fetal causes of growth retardation were excluded, the sensitivities of the pulsatility index in the internal carotid artery, the umbilical artery, and for the umbilical artery/internal carotid artery ratio were 65, 83, and 88% at the 1 standard deviation (SD) cutoff level; and 48, 60, and 70% at the 2 SD cutoff level. Growth-retarded fetuses with structural or chromosomal defects had normal pulsatility index values in the internal carotid artery.