Maximum flow velocity waveforms were recorded in a longitudinal study from the fetal ascending aorta and fetal pulmonary artery in 46 normal pregnancies and, in addition, from the umbilical artery in 21 cases of intrauterine growth retardation between 19 and 33 wk gestation. In normal pregnancy, the mean peak systolic velocity (PSV) in the ascending aorta increased from 49.4 cm/s at 19 wk of gestation to 79.0 cm/s at 33 wk of gestation. The corresponding increase in PSV in the pulmonary artery was from 39.0 to 63.7 cm/s. The ratio for the PSV between the two arteries remained constant (1.25-1.29). Mean values of PSV in both arteries were linearly related to gestational age. Normal limits according to age were constructed by establishing the 5th and 95th percentiles. In intrauterine growth retardation, the PSV in the pulmonary artery was decreased (<5th percentile) in 95% of cases, PSV in the ascending aorta was reduced (<5th percentile) in only 57%. No relationship was established between PSV in both arteries and the presence or absence of end-diastolic flow velocities in the umbilical artery. The outcome of fetuses with intrauterine growth retardation, as expressed by Apgar score at 1 min and umbilical cord pH, bears no relationship to the PSV in ascending aorta and pulmonary artery.