To study the in vitro anticoagulant activity of cyclodipeptide(cyclo-(Tyr-Leu),cyclo-(PhePhe) and cyclo-(Phe-Tyr)) isolated from Sparganium stoloniferum Buch.-Ham. METHODS AND RESULTS:The special kits were used to measure anticoagulant activity with platelet poor plasma collected from abdominal aorta of male SD rats,including prothrombin time(PT),activated partial thromboplastin time(APTT) and thrombin time(TT).PT,APTT,TT were prolonged by the addition of cyclo-(Tyr-Leu),cyclo-(Phe-Phe) and Cyclo(Tyr-Phe). The former two presented no dose-effect relationship but the latter did. CONCLUSIONS:Of three cyclodipeptides from S. stoloniferum,all the three show anticoagulant activities with cyclo-(Phe-Tyr) being by far the most effective.