After the injection of DDAVP in 39 non-anesthezised dogs (0.4 μg/kg) there was an average increase of factor VIII:C activity up to 145% (p<0.0001) and of fibrinolytic potential of euglobulin precipitate (t-PA) up to 196% (p<0.0001). The injection of DDAVP was repeated in each dog of a group of good responder animals at weekly intervals , but after : -A) Pentobarbital anesthesia (30 mg/kg)- the increase of factor VIILC was reduced from 164% to 116% (n=11; p<0.0005) and the increase in t-PA was

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... duced from 270% to 192% (n-11; p<0.05). -B) Injection of propranolol (tmq/kq) - the increase of factor VIILC was reduced from 167% to 110% (n=13; p<0.0005) and there was no decrease of fibrinolytic activity, (n-13; n.s.). -C) Splenectomv-the increase of factor VIILC was reduced from 166% to 122% (n=10; p<0.0005) and t-PA was increased from 196% to 256% (n=9; n.s.). There were no statistical significant differences in factor VIILC and fibrinolytic activities after repeating only the injection of DDAVP three times in the same animal at weekly intervals (n=5; n.s.).We conclude that the increase in F. VIII:C and fibrinolytic activities observed after DDAVP infusion are due to different mechanisms of action. On the one hand, pentobarbital anaesthesia reduced the increase of factor VIILC and t-PA , but on the other hand , beta-blockade and splenectomy influenced differently the behaviour of both biological activities.