Studies on coagulation and fibrinolysis were performed on 40 healthy women who underwent an uncomplicated pregnancy. From each subject, samples were taken 6-10 times during pregnancy, and all the changes were related to the subjects' own 6-week postpartum values. Increased concentration of coagulation factors II-VII-X was revealed by Normotest. The incidence of cold activation of factor VII increased from 14% in non-pregnant women in these series to 94% in pregnant women near term. A marked increase in the proteolytic capacity was probably an effect of increased plasminogen concentration and decreased antifibrinolytic activity. The latter did not, however, parallel the immunological determination of the antiplasmins. The concentrations of antithrombin III and CI inactivator of complement both decreased during pregnancy. It is concluded that the overall effect of these changes is probably in favour of coagulation.