Summary.  Background:  Pregnancy, oral contraceptive (OC) use and hormone replacement therapy (HRT) are established risk factors for venous thrombosis. Acquired resistance to activated protein C (APC) has been proposed to contribute to the increased thrombosis risk. Mouse models are often used for preclinical testing of newly developed hormone preparations. However, it is not known whether hormone-induced APC resistance is also observed in laboratory animals. Objectives:  To investigate whether hormonal changes modulate APC resistance in mice, we used pregnant mice as a model of hormone-induced APC resistance. The effect of pregnancy on APC resistance was studied in wild-type and factor (F)V Leiden mice. Methods:  APC resistance was determined in mouse plasma using a thrombin generation-based APC resistance test. APC resistance determinants, i.e. prothrombin, FV, FX, antithrombin and protein S levels, and of tissue factor pathway inhibitor (TFPI) activity were evaluated in plasma from non-pregnant and pregnant mice. Results:  In contrast to humans, pregnancy induced a decrease in APC resistance in wild-type and in FV Leiden mice. Pregnant mice had higher levels of prothrombin, FV, FX, protein S and TFPI activity as compared with non-pregnant mice. Conclusions:  Pregnancy causes a decrease in APC resistance in mice, which can be explained by the elevation of protein S levels and increased TFPI activity in plasma. Our findings show species specificity in the effects of pregnancy on the major determinants of the protein C system and suggest that protein S and TFPI play an important role in the development of pregnancy-induced APC resistance in humans.