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IT is now well established that progesterone can either facilitate or inhibit the effects of oestrogen on gonadotrophin release and behaviour¹. Whether facilitation or inhibition is observed depends primarily on the time interval between exposure to oestrogen and progesterone, but the mechanism by which progesterone exerts these effects remains unknown. Attempts to demonstrate progesterone receptor sites within the brain and pituitary have given equivocal results^2–10, leading to speculation that the central effects of progesterone may be mediated through a mechanism fundamentally different from that found in peripheral tissues^1,2,6. More recently, however, several studies have suggested that oestrogen-inducible progestin receptor systems similar to those found in peripheral progesterone target tissues can be identified in the brain and pituitary if sufficiently sensitive and specific experimental procedures are …