Treatment of mink by subcutaneous implantation of progesterone tablets, absorbed at the rate of 600–900 μg. daily, failed to prevent delay in implantation of the embryo. The treatment may possibly have resulted in increased productivity. The factors regulating the time of implantation are discussed. Considerations of quantitative ovarian hormone requirements and interactions in other species make it appear probable that the failure was not due to too low a dosage. This would imply that, besides the presumed increase of luteal secretion, implantation requires the presence of an extra-ovarian factor acting upon the uterus, or passing in its secretion to the blastocyst.