We studied the fibrinolytic enzyme system in non-pregnant female New Zealand rabbits infused with rabbit brain tissue thromboplastin for 2 h and in rabbits receiving infusion of isotonic saline. The rabbits treated with thromboplastin responded with statistically significant changes in haemodynamic variables, a decrease in plasma concentration of fibrinogen (P<0.01), a decrease in platelet concentration (P<0.01), a prolongation in prothrombin time and activated partial thromboplastin time (APTT) (P<0.01), consistent with an induced intravascular coagulation. The thromboplastin infused animals had furthermore changes consistent with activation and subsequent inhibition of the fibrinolytic enzyme system. The plasminogen activity dropped continuously during the 2-h infusion (P<0.01) along with the activity of α₂-antiplasmin (P<0.05). The activity of the urokinase-like plasminogen activator decreased slightly (P<0.05) during the 2-h infusion, while the factor XII-dependent plasminogen activator activity did not change. The tissue-type plasminogen activator activity decreased (P<0.01), and that of plasminogen activator inhibitor 1 (PAI-1) increased pronouncedly after 1 hour of thromboplastin infusion (P<0.01). We conclude that infusion of tissue thromboplastin causes activation of both coagulation and fibrinolysis and that tissue thromboplastin mediates a rapid inhibition of t-PA which is probably related to activation of platelets and release of PAI-1.