We investigated the acute hemodynamic effects of in utero fetal intravascular transfusion in anemic fetuses. A conductance catheter technique of measuring left ventricular (LV) volume and pressure-volume analysis was employed in six anemic ovine fetuses of 131 days gestation (range 131-133 days). The mean fetal weight at necropsy was 3,795 +/- 166g. An intravascular transfusion of 120 ml packed maternal red cells was given over 12 minutes (rate of 2.6 +/- 0.1 ml/kg/min) increasing the hematocrit from 22.5 +/- 1.3% to 41.8 +/- 0.8%. The infusion of packed red cells leads to an approximate increase in blood volume of 30%. Heart rate, left ventricular stroke volume, and contractility, as assessed by end-systolic elastance, did not change significantly with transfusion. Left ventricular afterload, as assessed by effective arterial elastance, increased from 11.4 +/- 1.4 to 18.6 +/- 2.8 (P < 0.01) mmHg/ml with transfusion, returning to baseline levels by 60 minutes posttransfusion. During the transfusion, end-diastolic pressure increased from 4.1 +/- 0.4 to 10.6 +/- 0.8 (P < 0.01) mmHg and end-diastolic volume increased from 2.0 +/- 0.3 to 2.9 +/- 0.5 ml/kg. However, by 60 minutes posttransfusion, end-diastolic pressure had returned to baseline levels, whereas end-diastolic volume remained elevated at 3.2 +/- 0.6 ml/kg. We conclude that the persistent increase in end-diastolic volume implies a possible increase in LV diastolic compliance.