RECENT experiments upon rats demonstrate that pregnancy can be prolonged by 4–10 days by any of the following treatments: implantation of anterior pituitary tissue (cattle), the injection of an alkaline extract of that gland, and of extracts of human pregnancy urine prepared by precipitation with barium-alcohol and/or phosphotungstic acid. Judged by the maternal weight curve, full development of the fœtus was reached at normal term, but the birth mechanism failed. Where pregnancy was prolonged for more than 3½ days, the fœtuses were invariably still-born; in several instances parturition was protracted for 12–70 hours. Since the ovaries of such animals were found, on biopsy, to be highly luteinised, prolongation was thought to be due to the persistence of the corpora lutea formed as the result of the treatment administered. Such was the view of Teel1 and of Levin, Katzman, and Doisy.2 There are, however, certain indications that another factor besides the corpus luteum is concerned in maintaining the conditions of pregnancy.