Individual fibrinolytic inhibitors (α2‐macroglobulin, antiplasmin, plasminogen activation inhibitors), their relative values and the variation of such values as well as that of plasminogen were measured in 120 women during normal pregnancy (40 in each trimester). α2‐macroglobulin and antiplasmin were increased throughout pregnancy. The inhibitors of plasminogen activation were determined by means of both a clot method and a caseinolytic method. The level remained largely unchanged during pregnancy. These findings were confirmed by the results obtained after gel filtration on Sephadex G‐200 of sera from 25 pregnant women. The level of plasminogen was found to be high during the whole of pregnancy. Our findings suggest that the reduced fibrinolytic activity during pregnancy is due to a decreased production and/or a defective release of plasminogen activators from the vessel walls.