Transforming growth factor-β (TGF-β) signaling in endothelial cells is able to modulate angiogenesis and vascular remodeling, although the underlying molecular mechanisms remain poorly understood. Endoglin and ALK-1 are components of the TGF-β receptor complex, predominantly expressed in endothelial cells, and mutations in either endoglin or ALK-1 genes are responsible for the vascular dysplasia known as hereditary hemorrhagic telangiectasia. Here we find that the extracellular and cytoplasmic domains of the auxiliary TGF-β receptor endoglin interact with ALK-1 (a type I TGF-β receptor). In addition, endoglin potentiates TGF-β/ALK1 signaling, with the extracellular domain of endoglin contributing to this functional cooperation between endoglin and ALK-1. By contrast, endoglin appears to interfere with TGF-β/ALK-5 signaling. These results suggest that the functional association of endoglin with ALK-1 is critical for the endothelial responses to TGF-β. © 2005 Wiley-Liss, Inc.