Summary.The permeability of the guinea pig placenta to I131‐labeled thyroxine and inorganic I131has been studied. The pertinent foetal and maternal samples were taken 3 to 4 ½ hr. after the administration of the radioactive compounds in question. The result goes to prove that thyroxine does not permeate the placenta in the first hours after its administration. Under similar test conditions more radioactivity was observed in the foetuses than in the mother animal after injection of inorganic I1