Radioactive zinc (65 Zn) uptake in the mouse fetus was studied as a function of time after administration. The fetal uptake of65 Zn increased markedly with time after administration to the pregnant mice of gestation period Day 17, i.e., from 0.28% of the dose 30 min after administration to 2.25% of the dose 24 hr after administration. The similar increasing tendency of65 Zn uptake was also observed in the fetal membranes, whereas the placental uptake decreased with time. The pregnant mice of Days 12, 15, 17, and 18 of gestation received a single dose of65 Zn, and the fetal uptake was observed daily until parturition. The fetal uptake of65 Zn increased with progression of gestation period after administration, and the65 Zn concentrations in the fetus exceeded those in the maternal blood. From these observations, it was assumed that there was no placental barrier against65 Zn transfer from dam to fetus in mice.

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