In order to examine the pharmacokinetics of vitamin K2, vitamin K2 (MK-4) was intravenously injected into mothers, and its transfer to placental tissue and the fetuses was examined. While incorporation of vitamin K2 into placental tissue was relatively active, transfer of vitamin K2 to fetal blood (cord blood) was small. So it was indicated that vitamin K2 incorporated into placental tissue from maternal blood is initially stored in the placenta and then gradually released into the fetal blood. Since vitamin K deficiency has been pointed out, release of vitamin K2 into milk was also examined. When vitamin K2 (MK-4) was injected into mothers, the release of vitamin K2 into milk increased with time even after the plasma vitamin K2 concentration in maternal blood decreased. So the presence of a vitamin K2 concentrating mechanism in the mammary tissue was indicated.