Transforming growth factor (TGF)-₁ is a potent stimulator of intimal growth. We showed previously that TGF-₁ stimulates intimal growth through early upregulation of plasminogen activator inhibitor-1 (PAI-1) and, subsequently, PAI-1-dependent increases in cell migration and matrix accumulation. We also showed that PAI-1 negatively regulates TGF-₁ expression in the artery wall. Here we use plasminogen-deficient mice to test whether TGF-₁-stimulated, PAI-1-dependent intimal growth and PAI-1 suppression of TGF-₁ expression are mediated through inhibition of plasminogen activation by PAI-1. We also use lineage tracing to investigate the origin of cells in TGF-₁-induced intimas. Surprisingly, both TGF-₁-induced, PAI-1-dependent intimal growth and PAI-1 suppression of TGF-₁ expression are independent of plasminogen. Moreover, approximately 50% of cells that migrate into the intima of TGF-₁-overexpressing arteries carry a smooth muscle lineage marker, <1% carry a bone marrow lineage marker, and the remaining cells carry neither marker. Therefore, PAI-1 stimulates intimal growth and suppresses TGF-₁ expression through activities other than inhibition of plasminogen activation. In addition, contrary to widely held models, our results do not support a role for plasmin (or thrombospondin) in TGF-₁ activation in the artery wall. Further identification of the molecular targets through which PAI-1 stimulates intimal formation and suppresses TGF-₁ expression in the artery wall may reveal new approaches for inhibiting intimal formation. Our studies also discount bone marrow as an important source from which TGF-₁ recruits intimal cells and suggest instead that TGF-₁ induces substantial cell migration either from the adventitia or from an extravascular, but nonhematopoietic source.