The results of determinations of some factors of the blood clotting and fibrinolysis systems carried out in 30 patients operated on under anaesthesia with neuroleptanalygesics are reported. The factors determined included: the time of spontaneous lysis of whole blood clot, the euglobulin lysis time, the fibrinolytic activity on Astrup plates, the fibrinogen level and platelet count. The purpose of this work was to demonstrate a possible action of neuroleptanagetic agents on the system maintaining the blood in equilibrium. The fibrinolytic activity was not increased but the fibrinogen level and platelet count were decreased. These changes, however, seem to depend on the effects of the surgical operation and not on the method of anaesthesia used.