Endothelial (e) nitric oxide synthase (NOS) activity modulates pulmonary vascular tone in the normal fetus and decreases pulmonary vascular resistance (PVR) at birth. Mechanisms contributing to sustained elevations of PVR and the failure of postnatal adaptation at birth are uncertain but may include decreased eNOS activity. To test this hypothesis, we studied the effects of chronic intrauterine pulmonary hypertension on lung eNOS content and NOS activity in an ovine model of perinatal pulmonary hypertension and in normal lambs. We measured eNOS mRNA and protein content by Northern and Western blot analyses, respectively. Calcium-dependent and total NOS activities were determined by assaying the conversion of L-[14C]arginine to L-[14C]citrulline from lung homogenates. To determine the effects of intrauterine hypertension on lung eNOS content, fetal lung tissue was harvested 8-12 days after intrauterine closure of the ductus arteriosus (DA) performed at 125-128 days of gestation (term = 147 days). Although positive immunostaining for eNOS persisted in lung vascular endothelium, eNOS protein content was reduced by 48%, as measured by Western analysis (P < 0.001). Chronic hypertension reduced lung eNOS mRNA content by 30% (P < 0.05). Compared with age-matched controls, Ca(2+)-dependent NOS activity was decreased after DA ligation by 75% (P < 0.01). We conclude that chronic intrauterine pulmonary hypertension decreases eNOS in the fetal lung. We speculate that decreased NO production contributes to failure of postnatal adaptation in this experimental model of persistent pulmonary hypertension of the newborn.