“…2b), it is possible that pituitary refractoriness may be brought about, at least in part, by previous exposure to increased levels of progesterone in the circulation. This does not exclude the possibility that this refractoriness may also be due partly to exposure to increased levels of other sex steroids, especially oestrogens, and to a rapid depletion of a readily releasable pool of LH. As in small rodents (Alleva & Umberger, 1966;Bingel & Schwartz, 1969;Norman, Blake & Sawyer, 1973;reviews: Everett, 1961reviews: Everett, , 1972Schwartz, 1969Schwartz, , 1973, spontaneous pre-ovulatory discharges of LH occur only during a restricted period of the day in hens maintained under constant lighting conditions (e.g. lights on from 07.00 to 21.00 h) (Wilson & Sharp, 1973).…”