Background: Sildenafil is a phosphodiesterase-5 inhibitor considered for antenatal use for a variety of indications. We sought to assess sildenafil pharmacokinetics in the pregnant ewe and fetus and evaluate its physiological fetal effects. Methods: Twelve fetal lambs (127–133 days GA, term 145) were chronically catheterized in utero. Ewes received different doses of sildenafil, either via subcutaneous injection (1.6, 2.0 mg/kg/day) or intravenous (IV) infusion (3, 5, 7, 10, and 12 mg/kg/day). Maternal and fetal sildenafil concentrations and metabolic status (blood gas analysis) were measured at given intervals. The fetal heart rate, pulmonary blood flow, systemic and aortic pressure, and maternal uterine artery pressure were continuously monitored. Results: The transplacental sildenafil transfer was 2.9% (range: 1.4–7.8%), preventing attainment of fetal target concentrations without toxic maternal levels. IV sildenafil infusion induced an immediate, temporary, dose-dependent reduction of pulmonary vascular resistance (38–78%) and increased both pulmonary blood flow (32–132%) and heart rate (13–49%), with limited nonlinear dose-dependent effects on systemic and pulmonary pressures. Fetal and maternal blood gases and maternal uterine artery pressures were unaffected by sildenafil infusion. Conclusion: In sheep, transplacental transfer of sildenafil is extremely low. Though, minimal fetal sildenafil concentrations induce an acute transient pulmonary vasodilation, well-tolerated by the fetus and ewe.