Aqueous extracts of onion, garlic and ginger inhibited platelet aggregation induced by several aggregation agents, including arachidonate (AA), in a dose-dependent manner. While onion and garlic extracts were found to be weak inhibitors of platelet thromboxane synthesis, ginger extract inhibited the platelet cyclooxygenase products and this effect correlated well with its inhibitory effects on the platelet aggregation induced by the above aggregation agents. These two effects were dose-dependent. Although the three aqueous extracts inhibited the biosynthesis of 6-keto-F1 alpha in rat aortic rings from labelled AA, they did not reduce prostacyclin production from endogenous AA pool in aortic rings. Aqueous ginger extract was extracted into three organic solvents in order of increasing polarity (n-hexane, chloroform, ethyl acetate). An analysis of the n-hexane extract revealed at least three clearly separated TLC bands containing materials that inhibited platelet thromboxane generation simultaneously increasing lipoxygenase products (HETE). The results indicate that if the same were happening in vivo, onion, garlic and ginger could be useful as natural antithrombotic materials.