Recently generated caveolin-1 deficient mice (cav-1 ko) suffer from severe lung fibrosis with marked pulmonary hypertension and arterial hypoxemia and may therefore serve as an useful animal model of this devastating human disorder. Accumulating evidence strongly supports the negative regulatory influence of caveolin-1 on endothelial nitric oxide synthase resulting in a constitutive hyperactivation of the nitric oxide (NO) pathway in cav-1 ko. We therefore hypothesized that a disturbed NO signaling is implicated in the evolution of the adverse lung phenotype of cav-1 ko. For this purpose, cav-1 ko of 2 months age were compared with knockout counterparts experiencing 2-month postnatal NO synthase inhibition by N^G-nitro-l-arginine methyl ester (l-NAME) treatment. Chronic l-NAME administration prevented adverse lung remodeling in cav-1 ko. Furthermore, l-NAME donation led to a normalized oxygen saturation (91.5±1.8% vs. 98.5±2.3%, P<0.01, n=10-12), a marked decrease in right ventricular hypertrophy (LV/RV ratio: 4.0±0.3 vs. 2.7±0.3, P<0.01, n=10-12) and reductions of the elevated pulmonary artery pressure (40.2±3.1 mmHg vs. 26.3±4.6 mmHg, P<0.01, n=6). Collectively, these improvements resulted in an enhanced exercise capacity of l-NAME-treated cav-1 ko. Finally, we found evidence for enhanced oxidative stress in untreated cav-1 ko which was substantially reduced by chronic l-NAME administration to cav-1 ko. In view of these data, we speculate that a perturbation of NO signaling, together with enhanced O₂^- production originating from NO synthases, may play a pivotal role in the pathogenesis of the adverse pulmonary phenotype seen in cav-1 ko.