Platelet count, ADP-aggregation and adhesiveness of platelets, fibrinogen, FDP, and prothrombin time were examined in 50 patients who had undergone open cardiac surgery. Ticlopidine (250 mg/day) and Dipyridamole (1-2 mg/day) were administered prior to the extracorporeal circulation and the platelet inhibitory effects were compared. Platelet count was reduced markedly with the initiation of bypass and the low level was maintained until the 3rd postoperative day. Hyperthrombocythemia was found after a week. ADP induced platelet aggregation and platelet adhesiveness were markedly depressed by the ACC. Fibrinogen showed low level during the ECC but the hyper-fibrinogenemic state was observed in postoperative course. FDP showed an abnormally high value in nearly half the cases, particularly in the cases of valvular replacement. Ticlopidine markedly depressed the ADP-aggregation and Dipyridamole reduced platelet adhesiveness and thus both drugs showed a prophylactic effect in the reduction of platelets.