Introduction The time of this symposium coincides with the tenth anniversary of the structural characterization of gonadotrophin-releasing hormone (GnRH) (Matsuo, Baba, Nair, Arimura & Schally, 1971). There were then high expectations that GnRH would not only be of help in the treatment of infertility but, with the synthesis of inhibitory analogues, could also hold the key to new methods of contraception (Guillemin, 1972). It is appropriate that we should now consider this 'old' peptide in a new light because in spite of all the varied predictions no-one could have foretold some of the ways the story of GnRH has developed. For example, GnRH and its stimulatory analogues have been of only limited success in the treatment of infertility, but they now constitute our most promising approach to contraception. Studies of the way in which GnRH can change from having a stimulatory to an inhibitory effect on the pituitary gland and