A small piece of vascular tissue punched from a rat aorta is able to clot plasma. This coagulation process is promoted by blood platelets, especially after their activation. Thrombin, generated by this clotting process, plays a key role in vessel-wall induced platelet activation. Vascular prostacyclin inhibits vessel-wall-induced clotting of platelet-rich plasma, possibly by inhibiting platelet activation. Type and amount of dietary fats were shown to influence vessel-wall-induced clotting via at least four different mechanisms, namely: by modifying vascular prostacyclin formation; by affecting the clotting potency of the vascular tissue per sec; by an effect on some platelet property, probably connected with platelet activation; by influencing a plasma factor. Each of these mechanisms, as well as the nature of vessel-wall-induced coagulation, requires further investigation.