In vitro and ex vivo effects of 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione (diferuloylmethane, curcumin) and acetylsalicylic acid (ASA) on the synthesis of prostacyclin (PGI2) and on platelet aggregation has been studied in rat. Both drugs inhibited adenosine diphosphate (ADP)-, epinephrine (adrenaline)- and collagen-induced platelet aggregation in monkey plasma. Pretreatment with ASA (25-100 mg/kg), but not curcumin (100-300 mg/kg), inhibited PGI2 synthesis in rat aorta. In the in vitro system, too, curcumin caused a slight increase in the synthesis of PGI2, while ASA inhibited it. Curcumin may, therefore, be preferable in patients prone to vascular thrombosis and requiring antiarthritic therapy.