Ex vivo antiplatelet properties of Etersalate and its effects on serum thromboxane A2 (TXA2) levels and prostaglandin I2 (PGI2) generation are studied in human volunteers at two levels of oral dosing. Etersalate inhibits at the lower dosage platelet function and decreased TXA2 levels, but PGI2 generation from rat aortic rings is stimulated when incubated with plasma from Etersalate-treated donors. Blood coagulation parameters remain within normal values. Etersalate administration can act on platelet arachidonate metabolism at a different level than that of the cyclooxygenase pathway.