Thirteen healthy women (age 24-44 yrs) were studied during their menstrual cycle. Samples were taken 2-3 times weekly for six consecutive weeks. Plasma concentrations of oestradiol and progesterone were determined in each blood sample and only women found to be ovulatory were included. From the hormone data the cycles were divided in five phases (early follicular phase, late follicular phase, early luteal phase, late luteal phase and menstrual period). The samples for the coagulation and fibrinolysis assays as well as the venous occlusion (20 min) tests were drawn or performed between 8-9 a m after fasting for 8 hrs. The fibrinogen F VIII:C and AT III were assayed and did not show any variations through the study period. Neither were any differences found in platelet counts, platelet mean volumes or platelet function measured as platelet adhesion and plasma beta-thromboglobulin. The fibrinolytic activity after venous occlusion decreased slightly during the late luteal phase (phase 4) as compared with the other phases. Large individual differences were, however, seen and no statistically significant differences between the five different phases were found. The plasminogen activator inhibitor (PAI-1) varied during the cycle but in most individuals within the normal range. In 7/13 women substantial fluctuations of the fibrinolytic activities during the cycle were seen. Four women had a significant fall of the fibrinolytic activity after venous occlusion during the late luteal phase (phase 4) and 3 others during the menstrual phase (phase 5). No co-variation between the fibrinolytic activities and PAI-1 was found. Multiple regression analysis showed a co-variation between fibrinolytic activities and progesterone.