Topical superfusion by PAF-acether over a branch of the mesenteric artery of the guinea pig invariably results in the formation of a platelet thrombus, adhering onto a denuded area of the vessel wall, or at sites where local retraction of the endothelial cells has occurred. It is demonstrated that the application of exogenous PAF-acether triggers a process of local autogeneration and release of endogenous PAF-acether (or PAF-like substances), maintaining a long phase of sequential regrowth and embolization of thrombi, eventually leading to a complete occlusion of the arterial lumen. Extracts of Ginkgo biloba, administered intravenously or in topical superfusion, inhibit both exogenous- and endogenous-induced thromboformation.