The rôles of oestrogen and progesterone in the control of the release of luteinizing hormone (LH) required for ovulation in the laying hen have been extensively discussed (Van Tienhoven, 1961; Furr, Bonney, England & Cunningham, 1973), but firm conclusions have not been reached. In laying hens progesterone and LH levels in plasma rise simultaneously 4–7 h before ovulation (Cunningham & Furr, 1972; Furr et al. 1973) and administration of progesterone advances the time of ovulation (Fraps & Dury, 1943). Oestrone and oestradiol also rise at this time (Peterson & Common, 1972; Senior, 1973a), but oestrogens have only been shown to inhibit ovulation (Fraps, 1954, 1955). The purpose of the present work was to examine the temporal relationship between the levels of oestradiol and LH in plasma throughout the 12–14 h period before ovulation. Thirteen White Leghorn laying hens were caged singly in a light-proof room and exposed to 14