We performed studies to examine the mechanism responsible for the antithrombogenic effect of aspirin in experimental animals. We measured the effect of 10 and 100 mg/kg doses of aspirin, salicylate, or a combination of both on the accumulation of radiolabeled platelets onto injured carotid arteries in rabbits. The effects of these agents on thrombogenicity measured as platelet accumulation onto injured carotid arteries, were correlated with the ability to inhibit platelet thromboxane A2 and vessel wall prostacyclin synthesis. We found that a low dose of aspirin significantly inhibited platelet accumulation onto the injured vessels, while a high dose reversed this effect. Thromboxane A2 and prostacyclin production, however, were maximally inhibited in rabbits given either aspirin dose. The reversal of the antithrombotic effect of the low dose of aspirin by a high dose of aspirin was simulated by administering a combination dose of high-dose salicylate and low-dose aspirin. We concluded that the reversal of the antithrombotic effect of a 10 mg/kg dose of aspirin by a higher dose of aspirin could not be explained solely by the inhibition of PGI2 synthesis and was affected by the salicylate moiety of aspirin.