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Several 9-halogenated progestagens are of high biological activity in the ewe. Cronolone (9-fluoro-11β-hydroxy-17-acetoxyprogesterone), SC-21110 (9-chloro-11β-hydroxy-17-acetoxyprogesterone and SC-5848 (9-chloro-11β-hydroxyprogesterone) are indistinguishable from progesterone in priming the spayed ewe to respond to oestrogen and moreover are 20 times as potent as progesterone in this bioassay (Shelton, Robinson & Holst, 1967; Holst & Moore, 1968; R. Sinclair, personal communication). Cronolone has a similar potency in the inhibition of oestrus (Shelton & Robinson, 1967), and in a flock of ewes allows precise synchronization of oestrus and ovulation after termination of treatment (Robinson, Moore, Holst & Smith, 1967). These findings indicate that 9-halogenated progestagens are similar to progesterone in duration of activity and are of high potency. To study further the biological activity of these compounds, they were tested for ability to maintain pregnancy in rats.

Virgin female rats weighing 200–250 g. were housed in a constant temperature environment with free access to food and