Abstract: Rhynchophylline (Rhy)produced a potent inhibition of rat platelet aggregatin in ex vivo. TXA₂ generation in platelet rich plasma from rats treated with Rhy obviously decreased in the collagen-induced group, but was not altered in the AA-induced group. These results suggest that Rhy suppressed the release of AA from platelet phospholipids stimulated by collagen, and this may be one of the mechanisms related to its action on platelet aggregation. Pretreatment with ASA, but not Rhy, reduced plasma PGI₂ in rats. In the in vitro system, the PGI₂ synthesis in rat aorta was shown to be inhibited by ASA but was not by Rhy. Rhy significantly reduced the number of mice died due to thrombosis by platelet aggregates. ASA prevented collagen plus adrenaline induced thrombotic death while Rhy prevented both ADP and collagen plus adrenaline induced thrombotic death.