AIM: To observe the effects of cimetidine(Cim) on platelet function and thrombosis. METHODS: After incubated with Cim in vitro , rat platelets were activated with ADP or thrombin. The platelet aggregation, platelet malondialdehyde(MDA) formation, platelet intracellular free calcium([Ca 2+ ] i), and thromboxane B 2 (TXB 2) were measured. The effects of Cim on electric-induced thrombosis in rat carotid artery were examined. RESULTS: Cim potentiated ADP induced platelet aggregation , increased the thrombin induced [Ca 2+ ] i and MDA formation, decreased TXB 2. Also, Cim shortened the duration of electric-stimulated occlution time in rat carotid artery. CONCLUSION: Cim increased platelet function and accelerated thrombosis.