The effect of PGE2 on regional blood flows in the chronically catheterized near-term pregnant sheep was investigated using radioactive microspheres. The injection of 20 mug PGE2 per kilogram into the left ventricle of eight sheep resulted in no change in maternal brain and noncotyledonary uterine flow. The renal blood flow increased from 692 to 892 ml/min (P less than 0.004). The uterine blood flow decreased from 673 to 317 ml/min (P less than 0.001). The trium was bypassed by injecting 7 mug PGE2 per kilogram of sheep into a fetal venous catheter and permitting it to reach the placental vasculature after placental transfer. Eleven sets of observations were made in eight animals. We observed no change in the intrauterine pressure, maternal brain flow, and noncotyledonary uterine blood flow secondary to this procedure. The maternal renal blood flow changed from 592 to 669 ml/min (P less than 0.007). The uterine blood flow increased from 762 to 853 ml/min (P less than 0.02). The uterine vascular resistance decreased from 0.124 to 0.115 mmHg x min/ml (P less than 0.04). It was concluded that 1) PGE3 crosses the placenta quite readily, and 2) PGE3 causes dilatation of the maternal placental vascular bed.