In order to determine the extent of the placental transfer of Lithium ion, pregnant rabbits at 27 ~ 29 days of gestation, which has hemochorial placenta similar to the human placenta, received 2 mM/Kg of Li⁺ in the form of LiCl intravenously. Maternal arterial blood, placental sinus blood, fetal blood, amniotic fluid and maternal urine were drawn two hours after the single dose of LiCl. Concentrations of Li⁺, Na⁺, K⁺ and osmolarity were measured in plasma of collected bloods, amniotic fluid and urine. Followings are the results obtained. 1) Evident level of Li⁺ was detected in fetal blood, although fetal plasma concentration of Li⁺ found to be almost one third of maternal plasma. 2) Plasma concentration of Li⁺ in placental sinus blood was higher than that in fetal plasma but lower than that in maternal plasma. It means that downward concentration gradient of Li⁺ from mother to fetus was still remarkable two hours after the injection. 3) Significant level of Li⁺ was also detected in amniotic fluid. It seemed likely that Li⁺, at least in part, excreted by the fetal urinary tract. 4) There were no differences in Na⁺ and osmolar concentration between fetal and maternal blood. 5) From above results, it was concluded that Li⁺ may transfer across the placenta but limited passage capacity through placental barrier for Li⁺ is significant, beacause net transfer assumed to be going on even at two hours, at which time maternal equlibrium has been reached.