Tissue samples were obtained from 115 swine fetuses from 10 litters and analyzed for tissue-bound Hg 24 h after mothers were exposed to low levels of methylmercury by iv injection. Absorption of Hg by the fetus and placenta increased throughout gestation in concert with increasing fetal weight, as did fetal hepatic Hg. Fetal renal Hg increased throughout gestation, but the increase appeared to be much greater than would be expected on the basis of weight increase alone. Blood Hg concentrations did not change significantly. Fetal brain Hg content and concentration increased dramatically toward the end of pregnancy, the gestational period during which the rate of brain growth is greatest in swine. The finding that a period of increased Hg concentration in brain corresponded with the period of maximal brain growth velocity is particularly interesting because of the hypothesis that the brain is especially sensitive to nutritional and, presumably, toxicological perturbation while it is growing most rapidly.