SUMMARY The daily administration of 1 mg. cortisone did not induce ovo-implantation in rats ovariectomized or ovariectomized and adrenalectomized on the 3rd day of pregnancy and injected with 4 mg. progesterone each day. Oestrone administered in daily doses of 1 μg./animal, however, did induce nidation in ovariectomized progesterone-treated rats deprived of their adrenal glands.