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Fetuses and neonates are known to be highly susceptible to methylmercury (MeHg) toxicity, but little is known about the relative uptake of MeHg from blood to the developing brain. We measured time-course changes in mercury (Hg) concentrations in the brain of fetal, neonate, weanling, and adult rats after an injection of 0.08 μg (0.4 nmol) Hg/g MeHg. In the prenatal experiment, MeHg was subcutaneously injected to pregnant dams on embryonic days 17, 18, 18.5, 19, 19.5, or 20, and Hg concentrations in tissues were measured in both mothers and fetuses on embryonic day 21 (1 day before parturition). Brain Hg levels in fetuses peaked 2 days after injection and were approximately 1.5 times higher than in mothers. In the postnatal experiment, the same MeHg dose was injected subcutaneously to male rats on postnatal days 1 (neonates), 35 (weanlings), or 56 (adults). Mercury concentrations in tissues were measured 1, 2, 3, 4, 5, or 6 days after the injection. Brain Hg levels peaked most rapidly in neonates, and were approximately 1.5 times higher than levels in weanlings or adults. Throughout the examined period, peak Hg levels in the brain and the Hg brain/blood ratio 24 h after injection were highest in fetuses, followed by the levels in neonates, and decreased with life stage. These findings suggest that relatively higher brain MeHg uptake is an important factor in the vulnerability of fetuses and neonates to MeHg exposure.We measured time-course changes in MeHg uptake in rats of different life stages.Peak brain Hg level in fetuses was 1.5 times higher than in mothers.Peak brain Hg level in neonates was higher than in weanlings or adults.Hg brain/blood ratio 24 h after MeHg injection was highest in fetuses and neonates.