A group of compounds isolated from the medicinal plant, Santolina oblongifolia, have been investigated for their effects on the release of platelet cyclo-oxygenase metabolite thromboxane A₂ (measured as thromboxane B₂) from ionophore-stimulated human platelets. These compounds, which are dual inhibitors of cyclo-oxygenase (prostaglandin E₂) and lipoxygenase (leukotriene C₄) activity in vitro, are: apigenin, luteolin, quercetin, herniarin, scopoletin, scopolin and aesculetin. All compounds assayed presented a dose-related response to thromboxane B₂ release, with the percentages of inhibition being slightly lower than the reference drug, ibuprofen. Inhibition was more evident with the flavonoids. Our data support the inhibition of arachidonic acid metabolism as one of the mechanisms for which flavonoids and coumarins from S. oblongifolia exert their anti-inflammatory effect