Nitric oxide (NO) modulates pulmonary vascular resistance (PVR) in the normal fetus by increasing the cyclic guanosine 3',5'-monophosphate (cGMP) content of pulmonary vascular smooth muscle cells. Although several vasodilator stimuli, including acetylcholine, decrease fetal PVR through the release of endogenous NO, fetal pulmonary vasodilation is often transient despite prolonged treatment. Because cGMP is hydrolyzed and inactivated by cGMP-specific (type 5) phosphodiesterases (PDE5), we hypothesized that PDE5 activity contributes to high fetal PVR and limits the capability of the fetal pulmonary circulation to dilate or sustain vasodilation in response to cGMP-dependent stimuli. To test this hypothesis, we studied the hemodynamic effects of dipyridamole in 19 late-gestation fetal lambs. To determine whether dipyridamole-induced vasodilation is dependent upon basal NO release, we measured the response to dipyridamole before and after pretreatment with the NO synthase antagonist nitro-L-arginine (L-NA) in five fetal lambs. L-NA completely blocked dipyridamole-induced pulmonary vasodilation. To evaluate the effect of dipyridamole on pulmonary vasodilation due to the stimulated release of NO, we studied effects of prolonged intrapulmonary acetylcholine infusions, with and without concomitant administration of low-dose dipyridamole, in six fetal lambs. During prolonged (2-h) infusions, acetylcholine and dipyridamole individually caused transient pulmonary vasodilation. When administered together, pulmonary vasodilation was of greater magnitude and was sustained for the entire study period. To determine the effects of dipyridamole on endothelium-independent pulmonary vasodilation, we investigated the hemodynamic effects of inhaled NO (5 and 20 ppm) alone and in combination with dipyridamole during mechanical ventilation with low FlO2. The combination of dipyridamole with inhaled NO resulted in a greater degree of pulmonary vasodilation than that achieved with inhaled NO alone. We conclude that dipyridamole-induced pulmonary vasodilation is dependent on endogenous (basal) NO production and that dipyridamole potentiates vasodilator responses to endothelium-dependent and -independent dilators in the ovine fetal pulmonary circulation. We speculate that PDES activity opposes vasodilation and maintains high PVR in the normal fetal lung.