Apocynin is a constituent of root extracts of the medicinal herb Picrorhiza kurroa and has been shown to possess anti‐inflammatory properties. We investigated the effects of apocynin on the production of arachidonic acid‐derived inflammatory mediators by guinea pig pulmonary macrophages. Apocynin concentration‐dependently inhibited the formation of thromboxane A₂, whereas the release of prostaglandins E₂ and F_2α was stimulated. Apocynin potently inhibited arachidonic acid‐induced aggregation of bovine platelets, possibly through inhibition of thromboxane formation. The present results suggest that apocynin might, beside its therapeutic effects in inflammatory conditions when given in a root extract of P. kurroa, also be a valuable tool in the development of new anti‐inflammatory or anti‐thrombic drugs.