To investigate the role of adenosine formed extracellularly in vascular homeostasis, mice with a targeted deletion of the cd73/ecto-5′-nucleotidase were generated. Southern blot, RT-PCR, and Western blot analysis confirmed the constitutive knockout. In vivo analysis of hemodynamic parameters revealed no significant differences in systolic blood pressure, ejection fraction, or cardiac output between strains. However, basal coronary flow measured in the isolated perfused heart was significantly lower (−14%; P<0.05) in the mutant. Immunohistochemistry revealed strong CD73 expression on the endothelium of conduit vessels in wild-type (WT) mice. Time to carotid artery occlusion after ferric chloride (FeCl₃) was significantly reduced by 20% in cd73^−/− mice (P<0.05). Bleeding time after tail tip resection tended to be shorter in cd73^−/− mice (−35%). In vivo platelet cAMP levels were 0.96±0.46 in WT versus 0.68±0.27 pmol/10⁶ cells in cd73^−/− mice (P<0.05). Under in vitro conditions, platelet aggregation in response to ADP (0.05 to 10 μmol/L) was undistinguishable between the two strains. In the cremaster model of ischemia–reperfusion, the increase in leukocyte attachment to endothelium was significantly higher in cd73^−/− compared with WT littermates (WT 98% versus cd73^−/− 245%; P<0.005). The constitutive adhesion of monocytes in ex vivo–perfused carotid arteries of WT mice was negligible but significantly increased in arteries of cd73^−/− mice (P<0.05). Thus, our data provide the first evidence that adenosine, extracellularly formed by CD73, can modulate coronary vascular tone, inhibit platelet activation, and play an important role in leukocyte adhesion to the vascular endothelium in vivo.