Prolactin is the hormone principally involved with stimulating milk production. Therefore, at first sight, a role for prolactin in controlling gonadotrophin secretion may seem strange, especially as its involvement at the ovarian level has only been irrevocably established in rodents, particularly the rat (see McNeilly, 1984). However, in almost all species studied so far, the high levels of prolactin induced by suckling and essential for lactation are also associated with a reduction in normal gonadotrophin secretion, principally the pulsatile secretion of luteinizing hormone (LH), and as a consequence, a suppression of ovarian activity. A similar association between increased levels of prolactin and a reduction in both LH and follicle-stimulating hormone (FSH) occurs during seasonal infertility in male and female ungulates and in pathological hyperprolactinaemia in both men and women.