The antithrombotic effect of acetylsalicylic acid is intimately linked to its reactivity. The labile acetyl moiety irreversibly acetylates not only cyclo-oxygenase, but other biological components. The presence off ubiquitous esterases leads to the rapid disappearance of ASA from the body. The pharmacokinetics of ASA is affected by the dosage form used and the presence of food. Despite the absence of a definable relationship between plasma ASA levels and response, recent data would suggest a dose of about 0.5 mg/kg/day adequately suppresses platelet aggregation without affecting prostacyclin formation.