THIS report documents (1) production of fetal goiter when perchlorate is administered to pregnant guinea pigs, (2) failure of large doses of triiodothyronine to protect the fetus against perchlorate induced goiter, and (3) studies on the placental transfer of I131 labeled triiodothyronine. The findings bear upon the use of these agents in the treatment of thyroid diseases during pregnancy. In addition, they may be relevant to broader aspects of fetal-maternal interrelations.