AbstractThe distribution of mercury was studied in pregnant and fetal rats after administration at days 11, 13, and 19 of pregnancy. Methyl‐203Hg‐chloride was used as a tracer to determine amounts of mercury in brain, kidneys, liver, heart, placenta, and whole fetus. The fetal brain was shown to accumulate 19 times more of the amount in the fetus than did the maternal brain the amount in the pregnant female. In addition, the fetal brain concentration was 3.4 times higher than the maternal brain concentration. Clearance studies showed that the organ concentrations were variable with little change occurring in maternal brain and fetus. These results are consistent with sensitivity to mercury damage being greater in the fetal than the maternal br