Recent studies indicate that assembly of high molecular weight kininogen on its multiprotein receptor allows for prekallikrein activation. On endothelial cells, factor XII activation is secondary to prekallikrein activation and amplifies it. The immediate consequence of plasma prekallikrein activation is the cleavage of high molecular weight kininogen (HK) with liberation of bradykinin. Cleaved high molecular weight kininogen is antiangiogenic. Bradykinin stimulates tPA liberation and nitric oxide formation. In addition, formed plasma kallikrein promotes single-chain urokinase activation and subsequent plasminogen activation. Kininogens and their breakdown products also are antithrombins. The angiotensin converting enzyme breakdown product of bradykinin prevents canine coronary thrombosis. The author presents a new hypothesis for physiologic assembly and activation of the plasma kallikrein/kinin system and discusses its influence on vascular biology.