Because nitric oxide (NO) dilates vascular smooth muscle cells, a deficiency of endogenous pulmonary nitric oxide production by nitric oxide synthase (NOS) has been suggested to be involved in the pathophysiology of pulmonary hypertension in congenital diaphragmatic hernia (CDH). Our aim was to determine whether experimentally induced CDH in rats results in a decrease in the synthesis of NO in the lungs. Adult Sprague-Dawley rats were fed 300 mg/kg of nitrofen at 10.5 days' gestation. CDH, control, and sham (dosed with nitrofen, but without CDH) lungs were homogenized at full term (22 days' gestation) for measurement of NOS activity using the 14C-L-arginine to 14C-L-citrulline conversion assay. Western blot analysis with anti-endothelial cell NOS (EC-NOS) monoclonal antibody (mAb) was performed, and NOS expression was measured by densitometry. NOS activity was highest in the pulmonary parenchyma of control rat lungs (0.420 +/- 0.20 fmol/min/mg lung; n = 11), intermediate in sham lungs (0.370 +/- 0.010 fmol/min/mg lung; n = 14), and lowest in CDH lungs (0.300 +/- 0.04 fmol/min/mg lung; n = 12). NOS activity in the CDH and sham lungs was significantly lower than that of control lungs (P < .05). There was no difference in pulmonary NOS activity between sham and CDH lungs. NOS protein expression by Western blot analysis paralleled the observation for NOS activity in all groups, with the highest concentrations in controls, intermediate expression in sham lungs, and lowest expression in CDH lungs. Both NOS expression and NOS activity are significantly decreased in CDH rat lungs. Pulmonary hypertension in this model may be attributable to a deficiency of endogenous NO. This is the first reported study to suggest that decreased NOS activity may result in pulmonary hypertension in CDH.