Angiopoietin-1 (Ang1) is a ligand for the endothelial specific receptor tyrosine kinase, Tie2, that protects the adult peripheral vasculature from vascular leakage. We tested the hypothesis that increases in levels of Ang1 reduce blood-brain barrier (BBB) leakage in ischemic brain. Mice were subjected to embolic middle cerebral artery (MCA) occlusion. Recombinant adenoviruses expressing Ang1 (Ad-Ang1) or a control gene encoding green fluorescent protein (Ad-GFP), or recombinant Ang1 protein, BowAng1, was administered to mice before MCA occlusion. Regional cerebral blood flow (rCBF), the brain tissue content of Evans Blue, and ischemic lesion volume were measured. Serum levels of Ang1 (183+/-31.9 microg/ml, n=4) were detected in mice receiving Ad-Ang1 or in mice treated with BowAng1 (262+/-35.4 microg/ml, n=7) but not in the control mice (n=11). Six hours after MCA occlusion, mice receiving Ad-GFP (n=8) or control protein (n=7) showed large Evans Blue leakage in the ipsilateral hemisphere (0.46+/-0.05 or 0.55+/-0.16 ng/mg tissue) whereas mice receiving Ad-Ang1 (n=6) or BowAng1 (n=7) had significantly (P<0.05) less Evans Blue leakage (0.26+/-0.07 or 0.14+/-0.03 ng/mg tissue). Infusion of recombinant human vascular endothelial growth factor (rhVEGF(165)) to ischemic mice resulted in significant (P<0.05) increases in Evans Blue leakage (1.24+/-0.34 ng/mg tissue, n=7) compared with the control mice. In contrast, infusion of rhVEGF(165) in ischemic mice receiving Ad-Ang1 did not significantly increase Evans Blue dye in the ipsilateral hemisphere (0.22+/-0.06 ng/mg tissue, n=6). Moreover, 24 h after ischemia mice receiving Ad-Ang1 had a significantly smaller ischemic lesion volume (22.6+/-2.7%, n=8) than the lesion volume in mice receiving Ad-GFP (44.7+/-3.7%, n=8), although rCBF reduced to approximately 20% of the contralateral levels in both groups of mice 10 min after ischemia. Our data demonstrate that Ang1 reduces BBB leakage in ischemic brain and consequently decreases ischemic lesion volume.