PIP: To determine the changes that occur in blood coagulation and fibrinolysis in women receiving oral contraceptives, 23 women were studied before and at regular intervals after starting therapy with oral contraceptives. In 15 women, blood samples were drawn for analysis after 1, 3, 6, and 12 months of contraceptive usage. In 8 other women repeated analyses were performed during 2 tablet cycles. This group's data were compared to an untreated group. The women received a variety of oral contraceptive preparations. The parameters examined were: the number of platelets, platelet adhesiveness, fibringen, Factor II, Factor VII, and Factor X activities, Factor VIII, plasminogen and the antifibrinolytic activity of plasma. In the group of 15 women a significant increase in the plasminogen concentration occurred after 4 months. At 5-6 months, the number of platelets were significantly increased, but platelet adhesiveness decreased after 2-3 and 5-6 months of usage. Factors II, VII, and X activities increased after 2-3 and 5-6 months. Antifibrinolytic activity increased after 5 to 6 months. In the group of 8 women, considerable variation in the fibrinogen, plasminogen, and spontaneous fibrinolytic activity was observed, with maximal concentration observed during the middle of the tablet period and with minimal concentration before the start of the next period. When these cyclic variations were compared to the untreated group, it was discovered that more variation was found in the group receiving oral contraceptives.