Maternal oxygen administration combined with fetal ultrasound is used for testing fetal activity. To evaluate the effects of maternal oxygen administration on the ductus arteriosus (DA) and pulmonary circulation, we studied 8 near-term fetal sheep. During fetal surgery, catheters were inserted into the main pulmonary artery and the ascending aorta to measure pressures. An ultrasonic flow transducer was placed around the DA and the left pulmonary artery (LPA) to measure blood flow simultaneously. In 3 fetuses, the DA was formalinized in order to prevent its constriction. We performed 11 studies in 8 fetuses: 6 studies in 5 fetuses without and 5 studies in 3 fetuses with DA formalinization. 100% oxygen was administered for 30 minutes via a face mask to the maternal sheep. Hemodynamic data were measured continuously and the pressure gradient and resistance across the DA were calculated. Arterial blood gas samples were obtained before and after maternal oxygen administration. Hemodynamic changes were analyzed statistically within the group, and compared between the two groups with and without DA formalinization. Maternal oxygen administration increased fetal arterial oxygen tension from 21.0 ± 3.5 to 25.4 ± 3.3 mmHg in total. In the fetuses without DA formalinization, DA blood flow decreased significantly from 907 ± 123 to 647 ± 217 ml/min, while the LPA blood flow increased significantly from 33 ± 21 to 153 ± 61 ml/min. The pressures in the main pulmonary artery and the ascending aorta did not change significantly. The calculated resistance across the DA did not change significantly (from 0.003 ± 0.002 to 0.005± 0.002 [mmHg*min/ml]). In the fetuses with DA formalinization, hemodynamic changes were quite similar to those without formalinization. There were no significant differences in changes between the two groups. These data suggest that maternal oxygen administration decreases the DA blood flow and increases LPA blood flow in fetal sheep and that this redistribution of blood flow is due not to DA constriction, but to diversion of blood flow into pulmonary circulation as a result of pulmonary vasodilatation.