Thirteen intravascular transfusions were performed in 13 human fetuses who were anemic because of severe red-cell alloimmunization. To investigate the status of the umbilical and cerebral circulations by pulsed Doppler ultrasound, we studied the fetal middle cerebral artery (N = 13), internal carotid artery (N = 11), anterior cerebral artery (N = 11), and umbilical artery (N = 13) before, within the first 2 hours after, and the day after intravascular transfusion. The gestational age at the time of transfusion was 21-31 weeks (mean +/- SD 25 +/- 3.1). The fetal hematocrits before transfusion ranged from 12-32% (23.4 +/- 6.1), whereas the hematocrit after transfusion was between 25-42% (35 +/- 5). The net blood volume transfused (volume infused--volume removed) ranged between 7.5-31.0 mL (16.0 +/- 7.4). The hematocrit of the transfused blood varied between 68-81% (74 +/- 4). Repeated-measures analysis of variance indicated significant differences in the pulsatility index values of the four vessels studied. The same analysis indicated significant differences in the pulsatility index values at the three time points. Multiple comparison tests showed that the pulsatility index was reduced significantly immediately after transfusion for each vessel studied, but returned to pretransfusion levels by the next day. These data suggest a change in vascular impedance soon after transfusion as a consequence of direct intravascular transfusion.