Studies of L-thyroxine T4kinetics and placental transfer were conducted in pregnant rats during the last days of gestation using radioiodine labeled and unlabeled T4. The maternal T4secretion rate was 53.0 ± 6.9 ng/hr/100 g body wt. The placental transfer rate was 0.463 ± 0.196 X 10-3/hr at 18 days and 1.516 ± 0.147 X 10-3/hr at 20 days. From these results, we calculated that at 20 days of gestation, 80 ± 8 pg/hr were transferred to the fetus, less than 1% of the fetal production rate. Inasmuch as we have demonstrated that little placental transfer of T4occurred during these studies, we conclude that the fetal rat hypothalamo-pituitary-thyroid axis develops autonomously.SpeculationThe present data plus the fact that the rats are hypothyroid at birth support the concept of an autonomous development of the hypothalamic-pituitary-thyroid axis. It suggests that the rat can serve as a useful model for development of this axis in the human.