Using turbidimetry we found that trilobine inhibited ADP-induced platelet aggregation both in vitro and in vivo. Incubated with TrL 0.5, 0.75, and 1.0 mg.ml-1, platelet aggregation was inhibited by 38.2%, 68.2%, and 94.0% respectively. The inhibitory rates were 47.6% and 84.0% with TrL 20 and 40 mg.kg-1 ip respectively in vivo. The formation of platelet TXB2 was inhibited by 40% with TrL 20 mg.kg-1 ip in vivo, while the formation of carotid artery wall PGI2 was not affected. The production of TXA2-like substance was inhibited by 37%, 53%, and 78% with TrL 0.5, 1.0, and 2.0 mg.ml-1 respectively.