The effects of various flavonoids and related compounds isolated from the root bark of mulberry tree on rat platelet lipoxygenase and cyclooxygenase products formed from [1-¹⁴C] arachidonic acid were studied. Morusin was found to inhibit the formations of 12-hydroxy-, 8, 10-heptadecatrienoic acid (HHT) and thromboxane B₂ (cyclooxygenase products) more strongly than the formation of 12-hydroxy-5, 8, 10, 14-eicosatetraenoic acid (12-HETE) (12-lipoxygenase product). Oxydihydromorusin and kuwanon C were also found to inhibit the formation of thromboxane B₂ more strongly than the formations of HHT and 12-HETE. Mulberrofuran A inhibited the formations of HHT and thromboxane B₂, but it increased the formation of 12-HETE. Albanol B and mulberrofuran F did not affect arachidonate metabolism in rat platelet homogenates.