The effects of dietary linoleic acid on arterial thrombus formation in rats were compared with the inhibitory effect of intravenous or intraaortic administration of PGE1, a potent inhibitor of platelet adhesion and aggregation. The "rat aorta-loop" model proved to be a useful method to induce a stable thrombus, obstructing the aortae of standard-fed rats with an obstruction time (OT) of about 96 h. Increasing the amount of dietary linoleic acid from 2.5 cal% to 30 cal% doubled OT to about 200 h. A constant intravenous PGE1 infusion of 10 mug/h increased OT significantly from the control value of 126.6 h to 160.1 h. When the same amount of PGE1 was infused intraarteriallu, the increase in OT was higher and doubled to 255.7 h. Using the "Filtragometer method" - in which platelet aggregatibility is measured in flowing human venous blood - a significant decrease in platelet stickiness was found in a group of patients consuming 12 cal% linoleic acid as compared to a similar group of patients with only 4 cal% linoleic acid in the diet. As similar results were obtained in patients with diabetes mellitus and in survivors of a myocardial infarction, increasing the amount of dietary linoleic acid may be of great therapeutic value for patients at risk of arterial thrombotic processes. These data support the hypothesis that part of the excess of dietary linoleic acid can lead to increased PGE1 concentrations.