Action of nicotinic acid (NA) on components of fibrinolysis in vitro and in vivo and on fibrinolytic activity in vivo was studied. It is shown that NA represses action of inhibitors as well as of fibrinolysis activators. Interaction of NA with plasma in vitro is associated with enhanced fibrinolytic activity on account of inhibition of antiplasmins. Action of NA of fibrinolytic activity of blood during intravenous administration depends on dose. Small doses of NA induce slight activation of fibrinolysis due to binding of antiplasmins. Administration of high doses of NA induces activation of fibrinolysis through thrombogenesis involving central nervous system.