Sixteen normal human fetuses were studied at both 20-23 and 30-33 weeks of gestation to investigate whether hemodynamic changes in the heart and major fetal arteries occurred in response to maternal hyperoxygenation in normal pregnancy. Velocity waveforms from the mitral valve, aortic valve, middle cerebral artery, aortic isthmus and umbilical artery were recorded before and during maternal breathing of 100% oxygen. The cerebral artery pulsatility index (PI) increased with oxygen administration at the first examination (1.56 +/- 0.25, mean +/- 1 SD, to 1.72 +/- 0.28, p < 0.05). At the second examination this increase in cerebral artery PI was more consistent (1.89 +/- 0.43 to 2.22 +/- 0.43, p < 0.001) and there was also a decrease in the aortic isthmus PI (2.64 +/- 0.43 to 2.40 +/- 0.33, p < 0.01) during oxygen administration. The variables determined from the velocity waveforms of the umbilical artery, aortic and mitral valve did not show any changes with maternal hyperoxygenation. Maternal hyperoxygenation resulted in velocity waveform changes suggesting an increase of cerebral vascular resistance and a redistribution of blood flow from the brain to the vascular beds supplied by the descending aorta in the normal human fetus in the third trimester, but this response was less obvious at mid-gestation. The middle cerebral artery and the aortic isthmus seem to be suitable sites for verification of this response.