Bleeding induced by microvascular transection in the rabbit mesentery stops by the formation of a haemostatic plug. Normal platelets as well as the normal coagulation and fibrinolytic systems are essential for haemostatic plug formation, the initial formation being mainly ADP-dependent and the stability mainly an effect of fibrin formation. The difference in haemostasis between arterioles and venules was abolished by aspirin (Arfors et al. Scand. J. Haematol. 9, 322, 1972). In this study we have investigated the effect of indomethacin. As with aspirin, venular haemostatic plug formation time was shortened and plug stability increased. Local infusion of PGE1 into the cranial mesenteric artery significantly prolonged arteriolar and venular haemostatic plug formation time. Measuring blood flow velocity, vessel contraction and haemostatic plug volume makes it possible to determine the proportion of platelets participating in the formation of an effective plug in individual vessels. Platelet aggregability is significantly higher in plugs formed at injuries on the arteriolar side of the microcirculation than on the venular, but this difference is totally abolished after indomethacin. In conclusion the difference in haemostasis between arterioles and venules in this model can be explained by prostaglandin being formed in the mesenteric preparation.