One of the compensatory hemodynamic mechanisms seen in the anemic human fetus is an increased cardiac output. With Doppler techniques, cardiac output was measured in 21 fetuses before and immediately after 38 intrauterine transfusions for severe red cell alloimmunization. Umbilical venous pressures were measured before and after transfusion; amniotic fluid pressure was also quantitated. After subtraction of amniotic pressure, umbilical venous pressure increased by 1.7 +/- 2.8 mm Hg (p less than 0.01). Left and right ventricular output declined by 19% and 22%, respectively (p less than 0.001). Four factors are known to affect cardiac output: heart rate, cardiac contractility, preload, and afterload. Fetal heart rate and mean acceleration, a measure of myocardial contractility, were unchanged after transfusion. A calculated mean increase in the fetoplacental volume of 18% in conjunction with an increase in umbilical venous pressure would indicate that cardiac preload was increased. We propose that intravascular intrauterine transfusion leads to an increased cardiac afterload, possibly by increasing blood viscosity. The fetal heart responds to the increased afterload by a decrease in stroke volume, leading to a fall in cardiac output.