Administration of fluids intravenously prior to spinal and epidural analgesia in obstetrics is required to prevent maternal hypotension and fetal hypoxia. A colloid solution, such as hydroxyethyl starch (HES), might be preferable considering the capacity to stay intravascularly for a longer period. In this study the placental transfer of HES and the hemodynamic effects after infusion were investigated using a chronic maternal-fetal sheep preparation. Either 500 mL HES 10% or 750 mL lactated Ringer's solution (RL) was infused intravenously into the ewe over 30 min. Fetal and maternal blood were assayed for HES, blood gases, and acid-base status before and at regular intervals after infusion. Maternal and fetal cardiovascular variables were recorded continuously for 90 min. After HES infusion, maternal HES levels peaked at 30 min ranging from 6.9 mg/mL to 12.1 mg/mL and declined at 24 h to levels between 0.3 mg/mL and 2.8 mg/mL. Mean fetal HES concentrations remained below 0.3 mg/mL. Infusion of HES decreased hemoglobin (Hb) and plasma viscosity (PV) in the mother. Infusion of RL decreased Hb, but did not change PV. Infusion of HES significantly increased uterine blood flow (UBF), cardiac output (CO), total oxygen-delivery capacity, and uterine artery oxygen delivery. In contrast, infusion of RL did not significantly change these variables. Infusion of HES increases UBF, CO, and uterine and total oxygen-carrying capacity in the pregnant ewe. No significant transplacental transfer of HES was shown.