Smmary: The effect of pregnancy on the components of the fibrinolytic enzyme system was determined by serial observations on 10 healthy women during normal pregnancy, labour, and the puerperium. The The haemostatic mechanism in pregnancy appears to be altered towards an enhanced capacity to form fibrin and a diminished ability to lyse fibrin. These changes may be a physiological development to ensure the integrity of the foetal and maternal circulations and provide rapid and effective haemostasis in the uterus during and after placental separation. Nevertheless, the changes may also establish a vulnerable state for intravascular fibrin deposition. Introduction The fibrinolytic enzyme system or plasminogen-plasmin system has a physiological role complementary to the coagulation mechanism in maintaining the patency of the blood vessels by promoting the removal of intravascular fibrin. The behaviour of the fibrinolytic mechanism in human pregnancy is therefore of special interest because an increase in the concentration of certain coagulation factors, particularly fibrinogen, is known to occur as pregnancy advances. Fibrinolytic activity has been reported to be decreased in late pregnancy, but many of the previous reports are single observations obtained under varying conditions and conflicting results have been presented. The purpose of this investigation was to elucidate the changes in the components of the fibrinolytic enzyme system induced by pregnancy and parturition by serial observations on a group of healthy women followed throughout normal pregnancy, labour, and the puerperium.According to current concepts the coagulation and fibrinolytic systems may be in a state of dynamic equilibrium which keeps the vascular compartment intact and patent, the coagulation system laying down fibrin on the vascular endothelium to seal any gaps which may occur, and the fibrinolytic system removing such fibrinous deposits after they have served their