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 gonads have added a new dimension to our understanding of reproduction. For example, we now know that GnRH and its analogues can have a direct inhibitory action, both at the level of the pituitary gland and the gonads (Rippel & Johnson, 1976; Hsueh & Erickson, 1979a, b), receptors have been demonstrated for GnRH on the Leydig cell and ovary, and finally there is the recent discovery of a "gonadocrinin" or GnRH-like factor in the ovary and testis (Clayton, Harwood & Catt, 1979; Sharpe & Fraser, 1980a, b; Ying, Ling, Bohlen & Guillemin, 1981). It is the purpose of this paper to review the ways in which GnRH can be manipulated to produce antifertility effects and to evaluate the most promising approaches to contraception. There are 3 different ways of interfering with fertility by manipulating the GnRH stimulation of the pituitary gland: (1) GnRH can be prevented from reaching its pituitary receptors by neutralization of GnRH in the hypophysial portal blood by antibodies; (2) the GnRH receptors can be blocked by chemical antagonists of GnRH; and (3) GnRH agonists exert inhibitory effects when given chronically.