Progestin released from the ovaries of pregnant rabbits consists of 20α-hydroxypregn-4-en-3-one (20α-dihydroprogesterone) and progesterone (Hafez, Tsutsumi & Kahn, 1965; Okano, Matsumoto, Kotoh, Endo & Seki, 1966). Because corpora lutea (c.l.), follicles and interstitial tissue are normally present in the ovaries of pregnant rabbits, it has been difficult to determine whether interstitial tissue and c.l. both secrete both steroids (Dorrington & Kilpatrick, 1965), or whether, as is frequently assumed, 20α-dihydroprogesterone comes predominantly from interstitial tissue and progesterone from c.l. (Hilliard, Archibald & Sawyer, 1963; Hilliard, Spies, Lucas & Sawyer, 1968). Recently it has been shown that progesterone comes from c.l. and not from interstitial tissue by experiments using unilaterally X-irradiated ovaries of pregnant rabbits: these contained either c.l. and interstitial tissue or interstitial tissue alone (Keyes & Nalbandov, 1968). In this paper, further evidence of the site of progesterone secretion is reported from experiments using normal ovaries of pregnant rabbits.