Seven hundred micrograms of T4 were injected into the amniotic cavity 24 h before delivery of five pregnant women scheduled for elective cesarean section at term. T4, T3, and rT3 concentrations were measured by RIA in amniotic fluid obtained at the time of the injection and in amniotic fluid and cord serum samples collected at delivery. Iodothyronine concentrations also were determined on cord samples from 24 full term control infants. The geometric mean serum T4 concentration in the experimental infants was 27.2 micrograms/dl, almost 3 times that of the control population (10.3 micrograms/dl); serum rT3 concentrations were markedly elevated to a mean of 657 ng/dl, compared to 254 ng/dl in control infants. The mean serum T3 concentration was slightly but significantly increased to 61.3 ng/dl (control, 48.3 ng/dl; P less than 0.02). Amniotic fluid T4, T3, and rT3 concentrations all increased significantly. T4 injection into the amniotic fluid is an effective method of increasing fetal serum T4 concentrations. The preferential pathway of monodeiodination of the injected T4 in the human fetus is to rT3 rather than T3.