The biological activity of the natural somatostatin can be quantitatively and qualitatively altered by the substitution and/or the exclusion of some of its amino-acids. The most used synthetic analog, SMS 201-995, has a potent inhibiting effect on GH secretion, but is less effective on insulin and glucagon secretions. It is mainly used in Endocrinology for the treatment of acromegaly. It is also useful for inhibiting the inappropriate TSH secretion from a thyrotroph adenoma. The LH-RH agonists ast essentially by desensitizing the gonadotroph from the endogenous LH-RH. By sub-cutaneous, intra-muscular or nasal route, they allow to inhibit the gonadal functions in some hormone-dependent cancers and in true precocious puberty. More recently, they have been tested for the treatment of gonadotropic adenomas, where they may have sometimes a paradoxical effect. In combination with exogenous gonadotropins, they enhance the control of ovulation and are new valuable tools for IVF programs. Clinical studies with the LH-RH antagonists are just beginning. The long-acting bromocriptine affords a new alternative to the oral treatment of hyperprolactinaemia. Its suppressive effects lasts at least 35 days in normal subjects and 21 days in patients with macroprolactinomas.