The hormonal regulation of haemostasis is a problem which has not received much attention. The data concerning the influence of hormones from the hypothalamic-pituitary-thyroid axis are scarce, contradictory and based mainly on clinical observations. The objective of the current research is to study the influence of the Thyrotropin releasing hormone (TRH), Thyroid stimulating hormone (TSH), Triiodothyronine (T3) and Thyroxin (T4) on the activity level of the vitamin K-dependent plasma factors of blood coagulation--factor II (F II), factor VII (F VII), factor IX (F IX) and factor X (F X). This study was carried out on 40 male Wistar rats. The necessary quantity of blood was obtained by cardiac puncture under ether narcosis. The indicators studied were activated partial thromboplastin time (aPTT), protothrombin time (PT), F II, F VII, F IX and F X, and were determined by means of Diagnostica Stago tests and with the help of an automatic coagulometer. The hormones studied were: TRH (0.06 mg/kg b.w.), TSH (1 MU/kg b.w.), T3 (0.08 mg/kg b.w.) T4 (0.08 mg/kg b.w.) prolonged aPTT (p<0.001) and PT (p<0.001). TRH and T3 significantly reduced the activity level of factors II, VII, IX and X; T4 only reduced the level of F II (p<0.05), and TSH did not induce significant changes in the haemocoagulation factors studied. The TRH, TSH, T3 and T4 hormones, although elements of one and the same axis, have an ambiguous effect on the vitamin K-dependent factors of blood coagulation. The results obtained show that the determined changes in the activity levels of the vitamin K-dependent plasma factors of blood coagulation are undoubtedly related to the hypocoagulation observed in the intrinsic and extrinsic pathways under the influence of the hormones of the thyroid axis.