In 19 out of 20 human fetuses we recorded blood velocity waveforms in the renal artery by means of a duplex Doppler sector scanner. One fetus was at 22 weeks gestation, 7 fetuses were between 32 and 35 weeks, and 12 fetuses between 36 and 40 weeks. There was no evidence of intrauterine growth retardation and there was a normal quantity of amniotic fluid in all pregnancies. We also recorded waveforms in the umbilical artery, the descending aorta, and the internal carotid artery. The RI in these vessels showed normal values in all cases. There was no end-diastolic flow in four of the seven fetuses studied at 32 to 35 weeks gestation. The mean RI of fetuses with end-diastolic flow was 0.86 (SD 0.03). The interpretations of these results, and ideas for future studies are discussed.