Pulmonary vascular remodeling associated with pulmonary hypertension is characterized by media thickening, disordered proliferation, and in situ thrombosis. The p21-activated kinase-1 (PAK-1) can control growth, migration, and prothrombotic activity, and the hypoxia-inducible transcription factor HIF-1alpha was associated with pulmonary vascular remodeling. Here we studied whether PAK-1 and HIF-1alpha are linked in pulmonary vascular remodeling. PAK-1 was expressed in the media of remodeled pulmonary vessels from patients with pulmonary vasculopathy and was upregulated, together with its upstream regulator Rac1 and HIF-1alpha in lung tissue from lambs with pulmonary vascular remodeling. PAK-1 and Rac1 were activated by thrombin involving calcium, thus resulting in enhanced generation of reactive oxygen species (ROS) in human pulmonary artery smooth muscle cells (PASMCs). Activation of PAK-1 stimulated HIF activity and HIF-1alpha expression involving ROS and NF-kappaB, enhanced the expression of the HIF-1 target gene plasminogen activator inhibitor-1, and stimulated PASMC proliferation. Importantly, HIF-1 itself bound to the Rac1 promoter and enhanced Rac1 and PAK-1 transcription. Thus, PAK-1 and its activator Rac1 are novel HIF-1 targets that may constitute a positive-feedback loop for induction of HIF-1alpha by thrombin and ROS, thus explaining elevated levels of PAK-1, Rac1, and HIF-1alpha in remodeled pulmonary vessels.