-Effects of pregnancy and lactation on warfarin-induced changes in blood coagulationrelated parameters were examined in rats. Warfarin (0.5 mg/kg/day) was given orally to pregnant and nonpregnant rats for 3 days from gestation day (GD) 17 to 19 or to lactating and non-pregnant rats for 3 days from post partum day (PPD) 10 to 12. Blood samples were collected from the rats on the day following the last administration (GD 20 or PPD 13) to measure prothrombin time (PT), activated partial thrombo---changes were observed in lactating rats. The reduction of the anticoagulant effects of warfarin may parconcentration in lactating rats, respectively.