The formation of malondialdehyde, a metabolite of prostaglandin endoperoxides, was studied in platelet-rich plasma. Small amounts were produced in response to ADP, epinephrine, or collagen and larger amounts in response to thrombin. Arachidonic acid induced platelets to produce large amounts of malondialdehyde. Malondialdehyde formation by platelets was prevented by aspirin or indomethacin. The measurement of malondialdehyde in platelet-rich plasma may be used as an indicator of platelet prostaglandin synthesis, and is simpler than the measurement of prostaglandins.