The effects of progesterone (P4) used in physiological studies and in delayed parturition in reproductive engineering were examined. A dose of 0.25, 0.5, 1, or 2 mg of P4 was repeatedly administered to Jcl:MCH(ICR) mice on days 17 and 18 of gestation, and plasma concentrations of P4 were investigated. The P4 concentrations in mothers and fetuses after administration of exogenous P4 were no differences between doses of 1 and 2 mg. Jcl:MCH(ICR) mothers administered a P4 dose of 1 mg did not give birth. Therefore, we consider 2 mg of P4 is an overdose and that it is evident that a dose of 1 mg P4 is sufficient to induce delayed parturition.