Antithrombotic effects of water-extractive components (extracts) from heshiko and narezushi, traditional fermented aquatic products in Japan, were investigated in terms of blood coagulation and fibrinolysis by means of animal experiments. Blood clot formation was suppressed by administration of heshiko and narezushi extracts to both groups of Wistar rats fed artificial and high-fat experimental diets. Prothrombin time and activated partial thromboplastin time of plasma prepared from the rats, as indices of blood coagulation, were not prolonged by administration of heshiko or narezushi extracts. In contrast, the activities of plasmin and tissue plasminogen activator (tPA), key enzymes in the fibrinolytic system, were significantly increased by administration of heshiko and narezushi extracts. These results suggest that the anticoagulating effects of heshiko and narezushi extracts are associated with the promotion of the fibrinolytic system via enhancement of plasmin and tPA activities.