Relations between thyroxin and oestrogens are bibliographically reviewed and experimentally studied on rats (14 puberal, 14 impuberal and 13 castrated females were treated for 20 days with oestradiol benzoate, alone or associated with thyroxin, in daily doses varying from 50 to 200 gamma). Differences between animals treated with oestrogen alone and with oestrogen + thyroxin were studied on vaginal smear: acidophilic index, kariopicnotic index and cellular morphology show a clear inhibiting action of thyroxin on oestrogen. Mechanisms of this action are discussed and the conclusion is drawn that thyroxin inhibits directly the oestrogen at the level of the genital tract.