To determine whether endothelium-dependent pulmonary vasodilation is selectively impaired by chronic intrauterine pulmonary hypertension, we compared the hemodynamic effects of an endothelium-dependent agonist, acetylcholine (ACh), with an endothelium-independent agonist, atrial natriuretic peptide (ANP), before, during, and after development of pulmonary hypertension in five late-gestation fetal lambs. Pulmonary hypertension was produced over 9-12 days by progressive inflation of a vascular occluder around the ductus arteriosus. Age-matched fetal lambs (n = 5) without occluders served as controls. Mean pulmonary arterial pressure increased from 44 +/- 2 (baseline) to 65 +/- 4 Torr after 10-12 days of inflation (P < 0.05). Left lung pulmonary vascular resistance (PVR) increased from 0.52 +/- 0.06 to 0.72 +/- 0.11 Torr.ml-1.min over 10 days (P < 0.05). O2 saturation remained at > 40% during the study period. Although brief intrapulmonary infusions of ACh (1.5 micrograms over 15 min) lowered left lung PVR by 29 +/- 8% before ductus arteriosus compression, ACh-induced pulmonary vasodilation was absent after 9-12 days of pulmonary hypertension. In contrast, the vasodilator response to ANP remained intact throughout the study period. ACh- and ANP-induced vasodilation did not change with time in control animals. In five hypertensive animals delivered by cesarean section, inhaled NO (20 ppm) reduced left lung PVR from levels achieved during ventilation with 100% O2 alone (0.61 +/- 0.31 to 0.24 +/- 0.007 Torr.ml-1.min), increased arterial O2 saturation from 51 +/- 14 to 84 +/- 13%, and increased arterial PO2 from 29 +/- 11 to 106 +/- 34 Torr.(ABSTRACT TRUNCATED AT 250 WORDS)