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We studied 31 healthy volunteers with a normal menstrual cycle, who were divided according to their age into a group of younger women and a group of premenopausal women. During four phases parts of the menstrual cycle, blood samples were drawn for determination of serum fibrinogen, tissue plasminogen activator (t-PA) antigen, t-PA activity, t-PA inhibitor (PAI-1) antigen, PAI-1 activity, and euglobulin clot lysis. Blood samples for determination of serum estra­diol and progesterone levels were obtained dur­ing the follicular and luteal phase of the menstrual cycle. Then values of fibrinolytic parameters for individual menstrual cycle phases were com­pared within each group. In young women, PAI-1 activity determined during the luteal phase dif­fered significantly from PAI-1 activity measured during the follicular and ovulatory phase. As for other fibrinolytic parameters, no significant changes were found in either group. Statistically significant differences were established between young and premenopausal women as concerns their serum t-PA antigen, t-PA activity, PAI-1 antigen, PAI-1 activity and euglobulin clot lysis in all phases of the menstrual cycle. During the luteal phase, there was a correlation between serum fibrinogen and progesterone levels in the whole group and in the group of young women, while the premenopausal group showed a significant increase in serum estradiol which correlated with a decrease in PAI-1 antigen and PAI-1 activity. Both groups demonstrated a cor­relation between increased serum progesterone and elevated t-PA activity during the luteal phase. The results of this study stress the impor­tance of analysing hemostatic parameters on exactly determined days of the menstrual cycle. In conclusion, premenopausal women showed lesser fibrinolytic activity than young females. The drop in serum PAI-1 antigene and PAI-1 activity, occurring during the luteal phase, suggests that estradiol may have an important role in the regulation of blood fibrinolytic activity.