1. Platelets are needed for haemostasis. Platelets adhere to damaged tissue, activation being mediated by exposed collagen, thrombin and ADP. The activated platelets then release compounds that cause vasoconstriction, such as thromboxane A2, and activate the clotting cascade culminating in the formation of a stable fibrin plug. Overactive platelets, however, may play an important role in causing coronary vasospasm and acute myocardial infarction. For almost two decades it has been widely held that the substitution of vegetable oils for animal fats in the diet decreases the reactivity of platelets. A reappraisal of this view seems long overdue in the light of new knowledge concerning the control of platelet function.