GnRH (Gonadotrophin Hormone Releasing Hormone) is a decapeptide synthetized in the hypothalamus and acting on the pituitary gland to stimulate gonadotrophin synthesis and release: analogs of this hormone have been synthetized. They induce firstly a stimulating effect on gonadotrophin secretion (flare up effect) followed by an inhibition of the gonadotrops. Secondly, the secretion of steroids by the ovaries is reduced leading to a pharmacological castration. The properties of these analogs are used for the therapy of endometriosis, uterine fibroids, breast pathologies and in IFVET programmes. Researches are in progress both to isolate other molecules and to validate their use in other pathologies.