Dynamics of protein C concentration was studied in rat blood after administration of thrombin and thromboplastin. Administration of 0.5 ml 1% thromboplastin caused fast decrease of protein C concentration, down to 60% of the initial level, within 3 min, while activity of factor V reached the minimal rate (30%) within 5 min. Content of protein C returned to the initial level in blood within 2-2.5 hrs and of factor V--within 6 hrs. After administration of thrombin 3 NIH in content of protein C was decreased to 91.3% whereas heparin was released only after injection of 6 NIH. The data obtained suggest that the protein C system responded earlier to occurrence of thrombin in circulation as compared with the neurohumoral regulators of the anticoagulation system; the protein C system is one of primary mechanisms of the antithrombosis defence.