The purpose of this investigation was to determine the antiplatelet mechanism of gingerol. Gingerol concentration-dependently (0.5-20 microM) inhibited the aggregation and release reaction of rabbit washed platelets induced by arachidonic acid and collagen, but not those induced by platelet-activating factor (PAF), U46619 (9,11-dideoxy-9 alpha,11 alpha-methano-epoxy-PGF2 alpha) and thrombin. Gingerol also concentration-dependently (0.5-10 microM) inhibited thromboxane B2 and prostaglandin D2 formation caused by arachidonic acid, and completely abolished phosphoinositide breakdown induced by arachidonic acid but had no effect on that of collagen, PAF or thrombin even at concentrations as high as 300 microM. In human platelet-rich plasma, gingerol and indomethacin prevented the secondary aggregation and blocked ATP release from platelets induced by adenosine 5'-diphosphate (ADP, 5 microM) and adrenaline (5 microM) but had no influence on the primary aggregation. The maximal antiplatelet effect was obtained when platelets were incubated with gingerol for 30 min and this inhibition was reversible. It is concluded that the antiplatelet action of gingerol is mainly due to the inhibition of thromboxane formation.