Exogenous estradiol (E2) causes rapid or delayed vasodilation in different vascular beds. Although mechanisms through which E2 induces vasodilation are incompletely understood, E2 upregulates endothelial nitric oxide synthase(eNOS) under some conditions. In other settings, E2 causes acute vasodilation through nonnitric oxide mediated pathways. Because fetal estrogen levels rise in late gestation and are high at birth, it has been hypoothesized that E2 may contribute to the normal fall in pulmonary vascular resistance (PVR) at birth. However, whether E2 causes vasodilation or alters vascular reactivity in the fetal lung is unknown. To test the hypothesis that E2 causes fetal pulmonary vasodilation and enhances endotheliumdependent vasoreactivity, we studied the hemodynamic effects of acute E2 infusion into the pulmonary circulation of late gestation fetal lambs. To measure pressures, catheters were placed in the aorta (Ao), main pulmonary artery (MPA), left atrium, and amniotic cavity of late gestation (126128 d; term=147 d) fetal lambs. A left pulmonary artery(LPA) catheter was placed to allow local left lung infusions. An utrasonic flow transducer was placed around the LPA to measure blood flow. After at least 48 hours recovery, 17βE2 was infused into the LPA at three different doses (1, 10, 100 mcg). Hemodynamic measurements were made at baseline and every 10 minutes for 2 hours after E2, and then repeated the next day (1824 hours). LPA flow, MPA and Ao pressure, and PVR were not changed after E2 infusion. To study the effects of E2 on pulmonary vascular reactivity, acetycholine (ACh; 3.0 mcg/min × 10 min) was infused prior to and at two timepoints (2 hrs, 1824 hrs) after E2. AChinduced fall in PVR was not different at 2 hours or 1824 hours when compared to initial response before E2 administration. We conclude that in the late gestation ovine fetus, exogenous E2 does not change pulmonary hemodynamics or alter reactivity. We speculate that although prolonged exposure to high E2 levels may influence the pulmonary circulation, abrupt elevations of E2 levels do no acutely lower PVR.