We studied four fetal sheep at 114 to 139 days' gestation to evaluate the effect of acute placental embolization on placental perfusion, fetal oxygenation and acid-base status, and the umbilical artery systolic/diastolic ratio. Studies were performed after maternal anesthesia was induced in an acute model, with serial umbilical artery embolizations performed at 20 to 30 minute intervals. Fetal umbilical flow, systolic/diastolic ratio, and arterial blood gas levels were followed up. One animal maintained a normal systolic/diastolic ratio despite a 50% drop in flow after the first injection and pH of 6.91, and died after the second injection. The remaining three animals showed significantly decreased flow after the third injection (p less than 0.02), with no change in the systolic/diastolic ratio. The systolic/diastolic ratio increased significantly after the fourth injection (p less than 0.05) when flow had dropped even further. Corresponding decreases in PO2 and pH did not achieve statistical significance. The magnitude of all changes increased with subsequent embolizations. We conclude that in this model significant reductions in flow result in small increases in the systolic/diastolic ratio and that greater magnitude changes in the systolic/diastolic ratio appear only with severely diminished flow and PO2, when acidosis develops.