OBJECTIVE Our purpose in this study was to investigate the effects of rapid intravenous crystalloid infusion on placental implantation-site blood flow and oxygen delivery in the near-term pregnant ewe. METHODS Maternal left ventricular, femoral arterial and venous, and bilateral fetal hind limb arterial catheters were placed in nine near-term ewes 5 days before the start of the study. Maternal and fetal arterial blood gas values, maternal hemodynamic measurements, and maternal organ blood flows (microsphere technique) were obtained before and after the intravenous infusion of 2.0 to 2.5 L of normal saline solution. Myometrial and placental implantation-site vascular resistances and oxygen delivery were calculated. The t test for paired comparisons was used for statistical analysis, with p < or = 0.05 considered significant. RESULTS Significant postinfusion increases in maternal mean arterial pressure, placental implantation-site blood flow, and skin, skeletal muscle, and renal blood flows were recorded. In the six animals that demonstrated a fall in hemoglobin concentration, a significant increase in placental implantation-site oxygen delivery and a significant decrease in placental implantation-site vascular resistance were also seen. No significant changes were seen in myometrial blood flow or myometrial vascular resistance. CONCLUSIONS Rapid intravenous crystalloid infusion selectively increases placental implantation-site blood flow in the near-term pregnant ewe and may improve oxygen delivery to the fetus, especially if hemodilution occurs.