* Department of Physiology & Biochemistry, The University, Whiteknights, Reading, RG6 2AJ, and † Imperial Chemical Industries Ltd, Pharmaceuticals Division, Mereside, Alderley Park, Macclesfield, Cheshire, SK104TG (Received 4 April 1975) Fraps (1961) proposed that in the hen an 'excitation' hormone secreted by the developing follicle initiated the release of the luteinizing hormone (LH) required for ovulation. Since progesterone induced premature ovulation in the hen (Fraps & Dury, 1943) it was considered the most likely candidate for such a role. Evidence showing that plasma progesterone rises either immediately before, or coincidentally with, the plasma LH surge is consistent with this hypothesis (Furr, Bonney, England & Cunningham, 1973). It has been demonstrated recently, however, that oestradiol (Senior & Cunningham, 1974) and testosterone (Etches, 1974) are also increased before the preovulatory rise in LH, which means that they cannot be precluded from consideration. Certainly there is ample evidence that oestradiol induces the LH