Measurements of BEI, triiodothyronine I131 resin uptake (T3U) and thyroxine I131 resin uptake (T4U) were conducted in 15 women with uncomplicated pregnancies and in their infants at the time of delivery and values were compared with those of euthyroid adults. In 13 additional women at term (with uncomplicated pregnancies) given intravenous loads of 500-8000 [mu]g sodium L-thyroxine, BEI and T3U were measured before and 1 hr postloading and in maternal and cord blood specimens at delivery. In control maternal subjects mean BEI was significantly higher than in the newborn or in euthyroid adults. Newborn and euthyroid adult values did not differ significantly. Mean T3U and T4U in these subjects increased progressively from maternal to newborn to euthyroid adult groups. These data suggest that the normal gradient of placental thyroxine transport is fetal-maternal. After maternal thyroxine loading, newborn BEI and T3U increased progressively with increasing maternal thyroxine load and increasing diffusion time, but these increases were much less than in maternal sera even after 8000 [mu]g thyroxine loads and relatively prolonged diffusion time. It is concluded that maternal-fetal placental transport of thyroxine in human term gestation is limited by the increased concentrations of maternal thyroxine-binding protein as well as by an inherent placental impermeability to thyroxine.