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 ฯฎ 166g. An intravascular transfusion of 120 ml packed maternal red cells was given over 12 minutes (rate of 2.6 ฯฎ 0.1 ml/kg/min) increasing the hematocrit from 22.5 ฯฎ 1.3% to 41.8 ฯฎ 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 ฯฎ 1.4 to 18.6 ฯฎ 2.8 (P ฯฝ 0.01) mmHg/ml with transfusion, returning to baseline levels by 60 minutes posttransfusion. During the transfusion, end-diastolic pressure increased from 4.1 ฯฎ 0.4 to 10.6 ฯฎ 0.8 (P ฯฝ 0.01) mmHg and end-diastolic volume increased from 2.0 ฯฎ 0.3 to 2.9 ฯฎ 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 ฯฎ 0.6 ml/kg. We conclude that the persistent increase in end-diastolic volume implies a possible increase in LV diastolic compliance.