Angiopoietin-1 (Ang1) has key roles in development and maintenance of the vascular system. The ligand is a potent inhibitor of vascular leakage and suppresses endothelial apoptosis and vessel regression. Ang1 was originally identified as a ligand for the receptor tyrosine kinase Tie2. Recently however Ang1 has also been found to activate the related tyrosine kinase Tie1. The contribution of Tie1 to mediating the effects of Ang1 on endothelial function is not known. In this study we used an siRNA approach to investigate the relative importance of Tie1 and Tie2 in transducing the effects of Ang1 on monolayer permeability and induction of apoptosis in human endothelial cells. siRNA directed against either Tie1 or Tie2 suppressed expression of each respective receptor by more than 90%. Ang1 inhibited endothelial monolayer permeability and this effect was prevented by suppression of Tie2 expression. In contrast, Ang1 inhibition of permeability was not affected by suppression of Tie1 expression. The ability of Ang1 to inhibit induction of apoptosis in response to serum deprivation was completely blocked by suppression of Tie2 expression, but not diminished by suppression of Tie1 expression. Taken together these data demonstrate that Tie2 mediates the inhibitory effects of Ang1 on endothelial permeability and apoptosis. The data also demonstrates that Tie1 does not transduce anti-apoptotic or anti-permeability effects of Ang1 in endothelial cells.