SUMMARY Daily injections of a combination of 2·4 mg. of progesterone and 24 μg. of oestradiol benzoate for 12–20 days, beginning 7 to 22 days post coitum in mink, failed to hasten ovo-implantation and increase litter size. On the contrary, it caused most of the females to be barren in comparison to similar non-treated animals in the stock colony. Daily injections of a combination of 8 mg. of progesterone and 2 μg. of oestrone for 12 days beginning four days post coitum likewise failed to hasten ovo-implantation in the mink, as shown by the length of pregnancy. This latter treatment caused a reduction rather than an increase in number of live kits at birth in the females that had litters. Daily injections of 3 μg. of oestrone for 8 days beginning 9 days post coitum caused a prolongation of the period of gestation, and presumably of the period of delay in ovo-nidation, and a reduction in number of kits alive at 7 days.
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Cochrane et al. (1962) studied this question.