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Recent studies have shown that progesterone 'withdrawal' in the pregnant rat, which is essential for normal parturition (Nelson, Pfiffner & Haterius, 1930; Stucki & Forbes, 1960) and for lactogenesis (Kuhn, 1969a), is due to a decrease in the rate of ovarian progesterone secretion (Fajer & Barraclough, 1967; Hashimoto, Hendricks, Anderson & Melampy, 1968). This in turn is largely due to a rapid increase of 20-hydroxysteroid dehydrogenase activity in the ovary which redirects the metabolism of pregnenolone away from progesterone towards 20-hydroxypregn-4-en-3-one, which is secreted in its place but which is hormonally inactive (Wiest, 1968; Wiest, Kidwell & Balogh, 1968; Kuhn, 1969b; Kuhn & Briley, 1970). The present communication gives evidence for a similar change in pregnenolone metabolism near parturition in the mouse ovary.

Mice of strain LACA/B/AC (Oxfordshire Laboratory Animal Colonies, Bicester, Oxon.) were mated over a period of 1 or 3 days. Litters were born on day