Due to an increasing concern about plutonium release to the environment, an attempt has been made to redefine the mass effect for placental transport of plutonium in mice. BALB/C mice were dosed by tail vein injection late in pregnancy with three concentrations of 239Pu (IV) citrate. Forty-eight hours following injection the mice were terminated and tissues prepared for analysis. The tissue activity analyses indicate that as the dose to the pregnant mouse was increased, the fraction of that dose incorporated into fetal tissue decreased. Placentas and other maternal tissues followed a similar pattern. To make reasonable and realistic estimates of plutonium transplacental movement in man and other species at very low exposure levels, one must extrapolate from low dose data only.