In 17 of 30 post partum rabbits intrafollicular injection (ovary exposed through a flank incision) of extracts of urine of pregnancy luteinized mature ovarian follicles, corpora lutea being unilaterally produced in the absence of general humoral changes of sufficient magnitude to cause any discernible change in the follicles of the opposite, untreated ovary. The corpora lutea so produced suffered regression much earlier than do the corpora lutea of pregnancy or pseudo-pregnancy; no functional activity could be demonstrated in them.