It is well known that breast feeding increases the secretion of the prolactin hormone and inhibits ovulation in women. This prevents the onset of a new pregnancy too soon, and so breast feeding was used in the past as a method of contraception. In addition to this physiological condition, there are many other pathological conditions in which the production of prolactin is increased. Thanks to new research, the intimate mechanism of the hyperprolactinaemia alterations affecting reproduction in mice has been discovered.