By inserting a polyethene, loop-shaped cannula in the aorta of rats, a platelet thrombus can be induced which, after some time (the obturation time, OT), blocks the aorta completely. When rats are fed diets containing increasing contents of sunflowerseed oil (SO), the OT increases SO-dose dependent. There is only a small increase in OT on feeding increasing amounts of hardened coconut oil (HCO). When in diets containing 60 cal% fat, the HCO is replaced by SO at various levels, there is a positive, significantly rectilinear relationship between OT and SO-dose. These results indicate that dietary SO and the linoleic acid fraction in particular, decreases the thrombotic properties of rat platelets, possibly by way of an increased biosynthesis of prostaglandin E1 – a highly potent inhibitor of thrombocyte adhesion and aggregation.